Hello, I purchased my boat used several years ago and need to put the correct prop on the boat. Boat: 1987,Thompson Carrera Cuddy Cabin 19'8" The engine is the MCM 165 HP in-line 4 cylinder, Closed Coolent, Stater Flywheel. OutDrive Ser #0B609235 Transom Assy # 0B630906 Motor Ser # B647509 My mechanic tells me the gear ratio is 1:84:1 for the OD. The out drive numbers are barely legible. The top of the outdrive states Alpha I-165. So I think it is still the original OD. Currently I have a 13 3/4 X 17 P prop on the boat. Maximum RPM is 3486 (shop tach), Max speed is 28 MPH. I want to get the boat up to the max RPM range as stated on the engine to between 4200 and 4600. I need to increase my top speed. I do a lot of medium range cruising. I have tried several sized props, higher pitch but same dia. what came on this boat from Thompson ? Before I start buying props I thought I might try a 13 or 15-P. I do not live close to a Mercruiser dealer that has a prop exchange program.
You can also look at the page below from their website that might help. It refers to a publication they have that might help "Everything you need to know about propellers".
Thank you for your response. I tried that and the reply from mercury marine was go to your local dealer and contact your boat manufacturer. So since Thompson has been out of business, I thought I would give it a try on this site which I have gained a lot of knowledge from. Maybe I will resend my info and see if I get a different rep ! Thank you again !
Thank you for your response. I have power tilt/trim and trim tabs. I will double check on your suggestion this weekend. What do you know about larger diameter props say vs. the 13.75. I have noticed they are getting more into changing diameters as well as pitch. I have been reading a lot of info re: dia. + pitch but practical experience goes a long way
30 years ago I was an R&D engine for Mercury. You can do little with prop dia yourself. Merc can do a lot more than a boater with dia. They have that figured out.
Triming can easily increase engine speed 500 rpm, try that first. If it does not work, look at bottom for hook, a 1/4" in 3' can make a difference. Does the boat bow steerring @ wot? If it does it's hooked.
Just reread your story. You have a glass boat not wood. Probably not hooked. Have you weighted boat, could have water logged foam floatation?
Have you tried a fuel filter, plug wires? Have you run a compression check? How about timing and timing advance. Plugs can also do it. Does pulling one wire at a time change engine rpm on any cycl that an other. Does the throdle open the carb all the way.
Do you have the correct gear ratio in outdrive, there were several ratio from 1.5 to 2.2 maybe ever 2.5. If you ehgine was designed for use with a 2.0 and you have a 1.5 . Prop pitch would change by 1.5 / 2 = .75 16" pitch x .75 = 12" pitch. You are probably not that bad but you get the idea?
Your boat should run 49/45 maybe 50 trimmed to max if set up right.
If you run trimmed way under that would limit top speed to 28, especialy if you a a planing fin on outdrive.
I just calculated max speed for your prop @ 3486 rpm with 1.84 ratio, top speeed without slip is 30.17 mph less 10% slip = 27 mph???
Problem is not prop, either boat weight, bottom, or egine hp output, or trim adjustment. Has boat been left in water? or trailered, that could do it. Dirty bottom.
I replaced fuel filter and that's good, Will check timing and advance along with a new tune up including rotor cap, rotor, plugs. I should check the plug wires. The throttle is adjusted and the carb seems to be operating fine, dont think it needs a kit. Checked comp when bought and all cyl were good at that time. The boat and trailer weighed I belive weighed around 2200 pounds (Its been 3 years?) when loaded on the transport vessel. Boat bottom is clean (Lake Superior) super cold.and is stored October - April. Every fall the boat is pressured washed and covered, spring it gets a cleaning and wax job. NO fin on OD so will play around with the trim settings, maybe 30-32 mph is good for the 165 HP 4 cyl with this boat. I think maybe a 15 P prop will work as a I need a spare.
30/32 is slow way slow. Can you email me a picture? If boat, motor, and trailer weight 2200 it should run better than 40 with ease. jplatou10@verizon.net or call me at 832 473 8928
When you get in out again, slowing raise trim until prop looses bit (rpm jumps) then trim under a little, that should give you best mph.
