Join Date: Aug 2013
Thanked 19 Times in 19 Posts
Rep Power: 4
Re: The right prop for my boat
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.
Last edited by Jaramaz; 07-29-2014 at 02:42 AM.