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The right prop for my boat

2087 Views 10 Replies 7 Participants Last post by  PhilCarlson
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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Don't bother unless you like spending lots of $ on bronze, clean the bottom lighten the load may surprise you.

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 :eek:
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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?
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.

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.


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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.
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A flexofold? If so, I agree that it is the best prop. Morris uses them now.


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 :eek:
A flexofold? If so, I agree that it is the best prop. Morris uses them now.
Noted that, a nice prop on a nice boat :)

FoF are of course very good - came out first in the YM (I think it was) test couple of years ago. Gori is also good; strange that the danes are so good on props!

The folder can have a better shape because it does not have to make a compromise on drag

The folders REVERSE ability is tugboat like and you will be shocked
It took a couple years to get to haul-out but did it last summer. Ended up having the prop re-pitched and now make 6ks at 75-80% RPM (5.5 if a cleaning is due), and it has paid off considerably when the weather turned bad.

Again, thanks everyone for the input.
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