The right prop for my boat

Well





Being the proud owner of the best prop money can buy



on a 29' boat with a 24' LWL and and absolutely perfect bottom i find while the boat will do 6.5 knots under-power it becomes unhappy above 5.5

Phil,

as stated by Tommays 6 knots is a lot for a 26 ft boat. As you know, each boat has a theoretical max speed based on the waterline length (vmax is about proportional to sqrt (lwl) ). Going faster than this max speed requires a lot of horse power ....

Another factor is, one should not use more than 75-85% of max rpm (there is anbother thread on someone who did, read that).

It is actually rather easy to make a first calculation on prop size and pitch, there are many propellor calculators available on the net. Most prop manufacturers has one on their home page.

Not sure you stated what prop you have today, without that knowledge it is impossible to say anything about the potential for improvements. However, it is likely to equip a sailboat like yours with a 2-bladed folding prop. Folding props from the 1980-ies were actually not so efficient, new props are much better. (To re-pitch an old folding prop is not only meaningless it may also seriously damage the prop).
- with a 26 ft boat and a 18 hp engine, a 3 blade prop is not your first choise, 3-bladed foldings are much more expensive than 2-blade.
- a fixed prop will slow down your sailing, do not go there.

I just looked into one of the prop calculators and guessed your length in waterline to be 20 ft, and your gearbox ratio to be 2.62 (the most used gear ratio on the 2GM20, however there are two other possibilities) in forward.
I also guessed some other parameters, of lesser importance.

Result came as a dia of 17" and a pitch of 13". Bit surprisingly large dia, but on the other hand these dimensions are very usual.
If there is not space for this size, then one might consider 3-blade which would be 15" dia (still 13" pitch).
These numbers here should just be regarded as examples. You must check gear ratio, lwl, and what prop you have today (and available space) before going ahead.

Suggest you look around on internet for prop calculators, many are just a spread sheet, easy to use.

Best

J

Thanks everyone! Lots of very helpful input (including the "don't bother" because it caused me to really think thru why to drive on with it).

Some more info based on the comments:

-LWL is 20.25 so the math says hull speed 6.01 or so. I've held her to 6.1 for brief but sustained periods under sail.

-Rationale: It may seem like I'm only pushing for a fraction of a knot. However, I'm running at 90% RPM in calm conditions get over 5kts. That's OK in and out of the harbor, but not so much in the big water over distance or when the weather hits. I really want to get my best speed at a sustainable RPM (2500-2800).

-The Perfect pitch propeller is intriguing, and seems to have plenty of satisfied owners.

-She has a fixed prop now. I've not really considered a folding prop because of cost and am interested how much of a difference it really makes.

-I'll look at the transmission but I'm sure it's nothing out of the ordinary.