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  #1  
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The right prop for my boat

I'm pondering replacing my prop during the next haul out to get a bit more speed when motoring:

The boat: North star 26 (26ft, disp 4600)
The motor: Yanmar 2GM20(F) 16HP at 3200 RPM
The prop: 2 blades, not sure of the size/pitch

Hull speed is 6.0something kts by the equation and I've had her a little over 6 under sail.

When motoring at 3200RPM in calm conditions I get to about 5-5.2 kts. I'd like to make 6 kts in calm conditions without pushing much above 3000.

My ignorance on this topic is nearly absolute, so I greatly appreciate the thoughts and advice of those who know.

2 blades, different pitch, three blades?
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Re: The right prop for my boat

Don't bother unless you like spending lots of $ on bronze, clean the bottom lighten the load may surprise you.
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Re: 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
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If a dirty bottom slows you down what do you think it does to your boat
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Re: The right prop for my boat

The pitch on your existing prop can be altered a lot cheaper than buying a new one. You will need the help of a good prop shop to figure out the diameter and pitch you will need to achieve your goal. However, over the years, I've rarely gotten it right the first time and needed several repitchings to get it right. Finding another North star 26 with the same engine and asking the owner what his prop is might also help.
But really, is .8 of a knot really worth all this?
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Re: The right prop for my boat

i have a perfect pitch 2 blade prop & a yanmar 2GM20F engine. my speed is 5.5 kn at 2200 rpm and 6.8 kn at 3600 rpm. i usually cruise at 5.5 kn. my boat is a nimble 30 express ( canoe hull ) you can google perfect pitch propellor & see what it is.
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Re: The right prop for my boat

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

Last edited by Jaramaz; 5 Hours Ago at 01:42 AM.
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