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post #11 of 15 Old 04-24-2013
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Re: Islander 28 propeller location from shaft support?

Originally Posted by 510datsun View Post
I'm guessing you didn't read the first message I posted at the start of the thread, but I will recap it for you.

In my initial post at the start of the thread, I stated I have an electric motor. The HP rating for electric and combustion engines is different because of the character differences of the two. I have been told the electric motors are more like the diesel vs a gas, more torque, and lower RPM versus a gas motor. So, trying to us a general formula for a combustion engine I don't think would work. An electric 5 hp rated motor would be close to the out put of 10-15 HP gas, so I have been told.

I don't have a transmission, but my pulleys are 2:1 from the motor to the prop shaft. Would be hard to plug my set up in to the formula.

I also initially posted, I have an Islander 28, I'm guessing since you wanted a hull, I have a 1976, hull #23. I was just trying to get an idea of what the diesel owners of the 28 were running to compare their's to what I have, to see if I'm OK with what I have.

In my first post, I stated I have a 3 blade 12x10.

So, kinda back to my initial question at the first post.

What prop are the owners of Islander 28 with diesel engines using?

Just looking for a general feel from the Islander 28 community

Thanks for wanting to help out, says a lot.
Thanks for the recap, 510. It looks like no one from the I28 community is responding. I'll contribute what what estimates I can based on my I30 experience and my EE degrees and experience.

Before I get started, I want to say most emphatically that you are best off talking with a reputable company that makes propellers, but one should never place blind trust in any so-called expert. You should always have a good idea of what the answer should be. That's my motivation for finding an estimate.

I will assume the usual size diesel engine for the 7000 lb I28 is an 11 or 12 HP (remember, that's diesel only). I base that on the fact that my 8300 lb I30 has a Yanmar 13HP 2GM(F) in it, and the more usual Volvo for my model is also a 13 HP model. But what we really need to know is not horsepower though, but torque. Sadly, Yanmar does not publish torque curves for its engines. Without giving you all the boring details, I'll say that my estimate of maximum torque for the 2GM is about 18 ft-lb based on other engines of similar size and RPM range. I expect you'd want something similar if you were to get a diesel engine.

My boat came with a Michigan Wheel 13x10 2-blade prop. It got me up to about 6.5 kts at 3500 RPM WOT, so it was about right for my 18 ft-lb of torque coupled to its 2.21 drive ratio transmission. But I was never happy with the way it backed down so I recently purchased a 3-blade 13x7 Campbell Sailor and have been much happier. I gained a half knot for the same RPM at cruising speeds and reverse is now actually useful. Interestingly, West by North Enterprises recommends a 13x8 if you have the more usual 2.61 transmission with a Yanmar 2GM.

The next part of estimating can be tricky and depends heavily on what kind of electric motor you have and its specs. Five HP DC motors are fairly rare. Most controllable motors are A/C even when battery powered. Generally, when an electric motor is battery driven, the DC power from the batteries runs through an inverter and goes to a cycloconverter or some sort of modulator such as a pulse width modulator, so it isn't beyond the realm of possibilities that you have an AC motor.

So we need to know some basics. Do you know the brand and model of your electric motor so maybe I can find it in my literature? Failing that, is it AC or DC? Operating voltage is useful too, if you have it. Is it a standard induction motor (by far the most common type of AC motor), a synchronous motor (almost unheard of in variable speed applications), or what? It makes a difference because the torque curves for the different motor types are very different.

T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD

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post #12 of 15 Old 04-24-2013
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Re: Islander 28 propeller location from shaft support?

The I28 I was on had a Volvo MD7A, which Volvo rates at 9.6hp @2600 rpm fwiw. And I have a note on the old PHRF certificate that it was a 12" Martec 2-blade folder. Doesn't mean that was the best choice--but it did work well. No idea what the pitch on that was.

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post #13 of 15 Old 04-25-2013 Thread Starter
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Re: Islander 28 propeller location from shaft support?

Thanks for putting out your input and questions dacap06.

You are correct, I'm looking for an approximation to have an idea about the propeller size before going to a shop. I really don't trust "experts", when it comes to sales, I tried to be some what informed. I then will be prepared with answer question the shop might have, shows I'm informed, and hopefully not an easy mark for buying something expensive I don't need.

So, I have an AC motor, 48v system, and the motor 100 amp max, 3.5 kw, 5 HP at the shaft.
I kinda thought the propeller size would be a 12 diameter, 3 blade. I'm just wondering about what pitch to get. I did get a guy who told me 80% of the propeller diameter, not sure if the rule is for electric, diesel, or gas. Based on that fact, I would use a 12x10, 3 blade.

I got it in my head from some where, the the hull to blade clearance was to close with the 13 diameter, causing some turbulence and noise issues on the Islander 28.

Thanks for the help
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post #14 of 15 Old 04-25-2013
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Re: Islander 28 propeller location from shaft support?


Electric motors are are a really different beast when compared to diesel engines. Unlike diesels, you can't damage your electric motor by under loading it (e.g. under propping it) but you sure can by overloading it.

For the rest of this post, I will assume your boat has an induction motor. Wikipedia has a really nice write-up for induction motor basics.

Take a look at the section on torque. Those graphs shown are for polyphase motors rather than single phase, but the graphs for your motor will look similar. As you can see from the graphs, torque is determined by the difference in power supply frequency and motor RPM. That difference is determined by load on the motor. When load goes up, current goes up as speed slows down, to a point.

However, the "to a point" never comes into play in properly designed applications. In practice, you don't ever want slipping to be more than 5% of frequency or so. Otherwise, current goes up far enough that your motor overheats if you leave it that way very long. That means it really doesn't matter whether your induction motor is type A, B, C, or D (although I am betting on single phase type B). You will always operate it in the steep part of the torque curve to the far right of those graphs where slippage is small.


T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD

Last edited by dacap06; 04-25-2013 at 07:37 AM.
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post #15 of 15 Old 11-04-2013
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Re: Islander 28 propeller location from shaft support?

My I28 has a little Yanmar so I am still working out pitch, but wondering about max diameter.

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