thompsondockside.com

What is the approximate prop pitch range (reasonable upper and lower bounds) for a 1963 15 ft Thompson Sea Coaster with 1973 65 HP Evinrude? The prop diamater is 13".

Peter

17,19, or 21

More likely a 17 or 19

That is a lot of power for that boat.

Could push it 40 plus mph!

Thanks. It seems I should order the 17. The transom agrees with you that the Evinrude 65 is a lot of motor.

If your Seacoaster is a '63 I don't think it is a 15 footer... I have a 1960 which is 15' 10", and I think they got bigger in '62 and '63... Seems to me that they were around 16 and a half.

John... I think you can help me understand the relationship between prop diameter, pitch, engine rpm and boat speed....

I just got back from a vacation and I had my boat in the water... with three guys and two tanks of fuel, I could easily go 32 mph ... the lake had a slight chop, and I hate to run full throttle very long... The rpm was at least 5000... maybe even 5500, I don't remember. (pulling a tube really slowed me down to 28 )

I was using the prop that came with the boat, and thought it was a 10x11.. I just looked now and see that it was one of the optional props - 10 1/4 x 10.

Last summer I found a 10x12, the other optional one for 1961 and got it as a backup.

Since my Johnson owner's manual says that the motor puts out 75Hp @ 4500, and the full speed operating range occurs at 4000-5000, I am wondering what the best prop for my boat is... given that I have some room in the rpm range.

So, under which circumstances should I use which prop....?

I generally don't hit the throttle hard out of the chute, to be easier on the transom.... I put it at maybe 1/3 and let the engine speed pick up for a few seconds, then half throttle... the bow drops down pretty quickly at maybe 20 mph.

Any thoughts...? I know your Cruisers and Evinrude 75 is about the same vintage... I am not trying to be a racer, but just looking for a safe match for the motor and performance.

Thanks.

John Hart

John

I run a 12" on my 1960 Cruisers 202 16' with my 1960 75 Evinrude.

Even with a 12" my rpm is close to 5000 with a light load.

The basic problem is even though our boats are the same vintage and weights, bottom condition can greatly efffect hp requirements for a given boat. My bottom is straight, as I replace orginial keel with a 5 x6" laminated inner keel. My top speeed has been as high as 41 on the gps.

Best way to prop your boat is make sure that at wot rpm is nor over 5000 rpm, but higher than 4500 with a light load. Even though book says a min of 4000, I would keep rpm at a min of 4250 at wot. If you have a heavy load and all you can get is 4000 rpm, I would not run at wot very long. Your engine will probably live longer at 5000 rpm than it will at 4000 under full load. Do not lug your engine.

If tubing is only 28 mph @ ??? rpm with your 10" then you might have to use the10" for tubibg and a 11 or 12 for cruising.

I have not run my 75 for 2 years, I currently running my 1960 40, idles better and uses a 1/3 less gas. Top speed is 28 and has a very comfortable cruise at about 20 mph with 3 or 4 people.

Never have skied behind it, I also have a Ske Nautique for sking.

Thanks John... but I just want to have you clarify one more thing... although it is a little hard to measure, it looks like my prop diameter's are 10".... I have been thinking that the 10x refers to the diameter, and the 11 or 12 refers to the pitch.... I am thinking that a 12 is a steeper pitch than an 11 or 10. (although I can't really see the difference as I hold them next to each other.

You refer to using a 12" (inch) prop... I presume that you mean pitch, since I can't see that a 12" prop would clear the cavitation plate on the motor...

Thanks John.

12" pitch

all me if you want 832 473 8928