Hi--
I've recently got our old family Thomson Off Shore 19 back in the water after 6 years on a trailer. It's 18'8" and is powered by a 1990, 100 HP Mercury. I've had it up and running a few times, and once on a plane the handling is, in a nutshell, scary! It bowsteers, the stern slides during turns, and does not bank into a turn but rather heels to the outside. It also torques to the right a lot, leading to VERY uncomfortable left turning. I must run with the trim all the way "in" (down). The RPM increases and right-torque is less when I trim it out, but any more than a little bit and it cavitates, especially on turns.
I'm guessing that all this is related to outboard shaft length. It's a 20" shaft; the transom is 21.5" deep, so the cavitation plate is ABOVE the bottom of the stern by 1.5". On a flat-bottomed boat like this, how far below the bottom should the cavitation plate be? All information I can find is for V-bottom boats: the cavitation plate should be 0-2" BELOW the bottom of the V. Was this boat designed to run with an extra-long (25") shaft motor? I'm reluctant to cut the transom down for fear of weakening it. Any guesses on how a longer shaft will affect handling and performance would be appreciated.
Correct outboard shaft length for 1961 Off Shore?
Moderators: a j r, TDockside, Miles, Moderators
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Ward Strong
- Posts: 9
- Joined: Sat Sep 09, 2006 12:32 pm
- Location: Vernon, BC Canada
The problem is most likely a hook in the bottom, causing the bow to drop resulting in bow steer.
Solution get the bottom straight. Not easy, but might be able to do, by jacking keel against a straight I beam or other very strong and straight beam. Very much like straighting a steel shaft by over bending to correct. Unlike steel, wood will have to be wet and left several weeks or months. I;ve read about it, but never done it.
Get bottom straight then test to see how if cavitation problem still exsists. Before I would cut transom I would also try a cupped prop.
Solution get the bottom straight. Not easy, but might be able to do, by jacking keel against a straight I beam or other very strong and straight beam. Very much like straighting a steel shaft by over bending to correct. Unlike steel, wood will have to be wet and left several weeks or months. I;ve read about it, but never done it.
Get bottom straight then test to see how if cavitation problem still exsists. Before I would cut transom I would also try a cupped prop.
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Ward Strong
- Posts: 9
- Joined: Sat Sep 09, 2006 12:32 pm
- Location: Vernon, BC Canada
Very interesting you'd mention a hook in the bottom. Part of getting it back in the water was a new coat of bottom paint. When I lifted it off the trailer there's a distinct, though shallow, depression just before the transom-- perhaps only 1/8" if a ruler were held against it. I believe it was due to being trailered with the transom hanging off the trailer supports; all the tansom/motor weight was on the bottom plates and rearmost ribs. Is this the hook you're talking about? It's anchored now, so I won't be able to measure it until it's out of the water again, but would 1/8" cause much bow steering?
Thanks for the great tip!
Thanks for the great tip!
Yes it could. Is the hook 1/8" in one foot? Check the bottom with a 4' or 6' straight edge, that will tell the tale. If its only 12" long and the rest is straight, the 12" hook can be filled, most likely its 1/8 of hook per foot and its 3 or 4 feet long and that's a different story.
If it's hooked that bad, try straighting the keel or in worst cause replace the existing inner keel.
Never heard of it being done, but maybe a 4 to 6" wide flange 6 to 8' long bolted to the top of the inner keel could do the trick.
Engine height alone will not cause bow steer unless your engine has some sort of after market Doel Fin, Whale Tail, or other such planing device.
As for your torque over in turns, could be as easy as to much pitch in prop, but probably due to wrapped and twisted bottom side to side. Could also be due to over powering, a 1990 100 hp is equal to a 150hp rating back in 1960. Also newer props are bigger dia. which could put more rotational torque out your hull. If the lean is only one side this could be a factor. If the lean changes left to right as the engine is trimed, its prop torque.
What engine was on the boat 6 years ago?
If it's hooked that bad, try straighting the keel or in worst cause replace the existing inner keel.
Never heard of it being done, but maybe a 4 to 6" wide flange 6 to 8' long bolted to the top of the inner keel could do the trick.
Engine height alone will not cause bow steer unless your engine has some sort of after market Doel Fin, Whale Tail, or other such planing device.
As for your torque over in turns, could be as easy as to much pitch in prop, but probably due to wrapped and twisted bottom side to side. Could also be due to over powering, a 1990 100 hp is equal to a 150hp rating back in 1960. Also newer props are bigger dia. which could put more rotational torque out your hull. If the lean is only one side this could be a factor. If the lean changes left to right as the engine is trimed, its prop torque.
What engine was on the boat 6 years ago?
All the comments about the bad effects of hoging are correct. However I am concerned about your reference to "trimming in or out" ... if you are refering to the motor trim or "tilt" adjustment, be aware that tilting the motor in toward the transom will cause the bow to be driven down under power resulting in very scary handling. The motor should be tilted out so that it is vertical when the boat is up on plane. Tilting it out a little bit more than vertical will not hurt... but never tilted it in.
Joe
Joe
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Ward Strong
- Posts: 9
- Joined: Sat Sep 09, 2006 12:32 pm
- Location: Vernon, BC Canada
Hi everyone--
I have cross-posted this thread to the Woodenboat.com forum, and opinions there are also that a hook or hog on the bottom is the problem. Joe, I'm aware of the effect of trimming in, but as soon as I trim it out, the prop cavitates-- which is one reason I thought shaft length is the problem. It still is a problem, but based on what others say, I suspect about 80% of the handling issue is the bottom hook, and 20% is too short a motor shaft.
Solutions: 1) Lower motor-- Pipefitter at Woodenboat mentioned a transom jack, which would be a cheap fix. 2) Fix hog-- much more difficult; I've got some thoughts and will start a new thread on this. Again, I'm going to cross-post this thread to Woodenboat.com.
Thanks to everyone for your insights.
I have cross-posted this thread to the Woodenboat.com forum, and opinions there are also that a hook or hog on the bottom is the problem. Joe, I'm aware of the effect of trimming in, but as soon as I trim it out, the prop cavitates-- which is one reason I thought shaft length is the problem. It still is a problem, but based on what others say, I suspect about 80% of the handling issue is the bottom hook, and 20% is too short a motor shaft.
Solutions: 1) Lower motor-- Pipefitter at Woodenboat mentioned a transom jack, which would be a cheap fix. 2) Fix hog-- much more difficult; I've got some thoughts and will start a new thread on this. Again, I'm going to cross-post this thread to Woodenboat.com.
Thanks to everyone for your insights.
Ward
Here what's probably happening, as the hook or hog pushes the bow down, it lifts the stern and engine, at some point the prop cavitates.
Lowering the engine should help as a band aid fix, but really not the proper solution. Cupping the prop will help in this case to some degree. Also attaching a wider cavitation plate above the prop will also help. The larger cavitation plate will also, make boat respond more to engine trim. This is not good particularly if engine is trimmed under. Making the boat have more bow steer.
Proper solution is straighten bottom.
The transom height is measured perpendicular to the bottom, not along the face of the transom. For a 15 degree transom the difference in measurement would be about 3/4". Is the transom original? 20 3/4" along face and 20 perpendicualar to bottom.
Here what's probably happening, as the hook or hog pushes the bow down, it lifts the stern and engine, at some point the prop cavitates.
Lowering the engine should help as a band aid fix, but really not the proper solution. Cupping the prop will help in this case to some degree. Also attaching a wider cavitation plate above the prop will also help. The larger cavitation plate will also, make boat respond more to engine trim. This is not good particularly if engine is trimmed under. Making the boat have more bow steer.
Proper solution is straighten bottom.
The transom height is measured perpendicular to the bottom, not along the face of the transom. For a 15 degree transom the difference in measurement would be about 3/4". Is the transom original? 20 3/4" along face and 20 perpendicualar to bottom.
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Ward Strong
- Posts: 9
- Joined: Sat Sep 09, 2006 12:32 pm
- Location: Vernon, BC Canada
Thanks again John: I didn't know transom height is measured at the vertical. The face of the transom is actually just under 22". I'm guessing it's not original, since I can find no serial number anywhere in it. When it was on the trailer, the motor trimmed to vertical and a straightedge is held along the bottom, the cavitation plate is nearly 1 1/2" above the bottom. So yes, the bottom needs fixing but I also think the motor needs to be lowered.
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Ward Strong
- Posts: 9
- Joined: Sat Sep 09, 2006 12:32 pm
- Location: Vernon, BC Canada
Well here goes my $0.02 again... I picked up on the issue of keel configuration. The trailing edge of the keel must be tapered out to nearly zero at the transom otherwise you will produce cavitation. A reference I have suggests the taper should be 15 inches long minimum. The official OMC service manual calls for a 10 degree taper on the keel with a maximum thickness at the transom end of 1/2 inch. Also transom height, measured square to the bottom is 20 inch +/_ 1/2 inch for long shaft motors ( 15 inch for short shaft motors)
Joe
Joe
1961 Thompson handling and outboard
The HOOK is the root of the problem.
I have experience with this on a 1958 Thompson 17 foot.
The boat feels like it is steering on a pivot located forward forward.
The boat feels like it rocks from side to side, at speed (28 mph on up).
The hook is LIFTING the stern, shifting the trim loads forward onto the bow. Thompson boats are not forgiving in this. They have a wide stern which, if it gets distorted with a hook, makes drastic effects on operation and handling.
Measure and map out the problem, and fix it with appropriate means. This may solve all of your problems.
If work is still needed, understand that you are right, older (1960 era and older) long shaft outboards were 22 1/2 or 21 1/1 inch mounting, depending upon brand and so forth. Hence, the transom height on your Thompson. Around 1963 designs changed and everyone standardized on a 20" hight Long shaft motor.
The simplest fix, Mercury used to say "cut the transom down". You might not want to do this. To avoid transom cutting do these things instead.
First, be sure to use a performance type high-rake prop. These are said by the Bass Boat drivers to produce "bow lift" (you want that!). Of course the REAL physics behind it is that ALL props actually create LIFT at the stern, and lift the engine and transom straight up. A conventional, basic aluminum prop has the most of the Stern Lift. You don't want that! High Rake props like Merc Laser II or Evinrude Raker use swept back design with tapered "ears". This design makes much less virtical lift, also making much less tendency to ventilation. That is what you want! Also, the 4 blades and even 5 blade Hi-Five have less stern lift again, and less ventilation than the 3 blades, as long as you need it and can afford the cost.
If props don't get you where you want to be, then I would add "set-back".
Simplest set-back is aluminum spacers and longer bolts. Dont' make these yourself. They are special made, like from Bob's Machine and the like. They may be in some marine catalogs, and I think are in the Mercury Accessory Catalog (the 4" thick catalog at the dealer) Setting the motor back from the transom gets the motor deeper in cleaner water, and changes the leverage for even LESS stern lift again, a good thing.
The best performance would be from a set-back bracket. There are several designs around. Some are simple brackets with a few different bolt holes for mounting heights. Some are adjustable with a ratchet. A set-back bracket will put the motor back about 4 inches. This will definitely improve performance for all the right reasons. Also a bracket will let you lower the mounting hight an inch or so---- though I doubt you would need it with the set back. Set-back brackets are around $350 for a good one. Remember you don't want the hydraulic jack-plates like the 90 mph bass boats. Just a bracket that once you have it set up, you leave it alone.
The bracket and prop stuff are easy. Unfortunately the only hard part is running the credit card at--- 700 bucks or so if you get prop and bracket. the hard part, and absolutely vital is fixing the hook. Won't cost much in material, but will take time and patience. Plus you must get the trailer set-up so it will not happen again.
I remember the Thompson liked to have a couple of 2 x 4 bunks for about 4 or 5 feet, set halfway between the keel and the chine, supporting the back of the boat. Often the trailer had guiding rollers (like a easy-loader type roller trailer) keeping it centered on the keel rollers to get the boat aboard. There was a cam operated jack that then lifted the bunk up, and lifted the boat off those guide rollers, sharing the load between bunks in the back and keel rollers from midships forward. Well, we forgot to engage the jacks on the bunks one winter, so the boat rested on the rollers about 10 inches in from the transom, and got one of those "hooks". Not pretty, but we were able to just fair it in with some glass balloon filled epoxy. (WEST System)
Cheers and good luck!
I have experience with this on a 1958 Thompson 17 foot.
The boat feels like it is steering on a pivot located forward forward.
The boat feels like it rocks from side to side, at speed (28 mph on up).
The hook is LIFTING the stern, shifting the trim loads forward onto the bow. Thompson boats are not forgiving in this. They have a wide stern which, if it gets distorted with a hook, makes drastic effects on operation and handling.
Measure and map out the problem, and fix it with appropriate means. This may solve all of your problems.
If work is still needed, understand that you are right, older (1960 era and older) long shaft outboards were 22 1/2 or 21 1/1 inch mounting, depending upon brand and so forth. Hence, the transom height on your Thompson. Around 1963 designs changed and everyone standardized on a 20" hight Long shaft motor.
The simplest fix, Mercury used to say "cut the transom down". You might not want to do this. To avoid transom cutting do these things instead.
First, be sure to use a performance type high-rake prop. These are said by the Bass Boat drivers to produce "bow lift" (you want that!). Of course the REAL physics behind it is that ALL props actually create LIFT at the stern, and lift the engine and transom straight up. A conventional, basic aluminum prop has the most of the Stern Lift. You don't want that! High Rake props like Merc Laser II or Evinrude Raker use swept back design with tapered "ears". This design makes much less virtical lift, also making much less tendency to ventilation. That is what you want! Also, the 4 blades and even 5 blade Hi-Five have less stern lift again, and less ventilation than the 3 blades, as long as you need it and can afford the cost.
If props don't get you where you want to be, then I would add "set-back".
Simplest set-back is aluminum spacers and longer bolts. Dont' make these yourself. They are special made, like from Bob's Machine and the like. They may be in some marine catalogs, and I think are in the Mercury Accessory Catalog (the 4" thick catalog at the dealer) Setting the motor back from the transom gets the motor deeper in cleaner water, and changes the leverage for even LESS stern lift again, a good thing.
The best performance would be from a set-back bracket. There are several designs around. Some are simple brackets with a few different bolt holes for mounting heights. Some are adjustable with a ratchet. A set-back bracket will put the motor back about 4 inches. This will definitely improve performance for all the right reasons. Also a bracket will let you lower the mounting hight an inch or so---- though I doubt you would need it with the set back. Set-back brackets are around $350 for a good one. Remember you don't want the hydraulic jack-plates like the 90 mph bass boats. Just a bracket that once you have it set up, you leave it alone.
The bracket and prop stuff are easy. Unfortunately the only hard part is running the credit card at--- 700 bucks or so if you get prop and bracket. the hard part, and absolutely vital is fixing the hook. Won't cost much in material, but will take time and patience. Plus you must get the trailer set-up so it will not happen again.
I remember the Thompson liked to have a couple of 2 x 4 bunks for about 4 or 5 feet, set halfway between the keel and the chine, supporting the back of the boat. Often the trailer had guiding rollers (like a easy-loader type roller trailer) keeping it centered on the keel rollers to get the boat aboard. There was a cam operated jack that then lifted the bunk up, and lifted the boat off those guide rollers, sharing the load between bunks in the back and keel rollers from midships forward. Well, we forgot to engage the jacks on the bunks one winter, so the boat rested on the rollers about 10 inches in from the transom, and got one of those "hooks". Not pretty, but we were able to just fair it in with some glass balloon filled epoxy. (WEST System)
Cheers and good luck!