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Islander 28 propeller location from shaft support?

4K views 14 replies 5 participants last post by  1217sailorbob 
#1 ·
I'm going to replace my prop shaft, its bent, worn in grooves, and appears to short. My prop is within an inch or two of the shaft support. I bought the boat with an electric motor, and the prop shaft has most likely been moved forward to connect to the electric motor or the shaft may have been shortened.

So, I know prop to hull clearance is important to performance and noise level. I have a 3 blade, 12x 10. The electric motor likes props similar to diesels.

I was going to get a new shaft and prop set up.
So, I was wondering where most if the 28 guys had their props sitting from the support and rudder?

What size and type prop the diesel guys have?

Your input would be greatly appreciated
 
#2 ·
WOW, no help.:confused:

This is why I have posted detailed information and pictures of important fixes for the Islander 28, seems help is most likely non-existent or long time coming.

I guess another area I will need research and post my findings.

While there are plenty of places to get information about propeller to hull clearances and distance from rudder, every boat is different with how they respond to certain changes.

There are fewer Islander 28's around then Catalina 27, so just takes longer to get the help, I'm OK with that.

I like a small cozy community, or a Catalina owner saying," nice boat, who made it, looks like the lines of a boat during the late 80's, what year is it?, what? a 1976? Where can I get one?

So, still waiting for help.
Thanks :D
 
#6 ·
Prop size and pitch involves calculations using the engine HP/torque, the transmission ratio, hull, and number of blades. You could expand the pool of people who might be able to answer if you could give values for all the variables.
 
#7 ·
Don't remember a damned thing except it was a Martec 2-blade folding prop, and unless memory fails me, we had room for two good sized zincs on the shaft in between the prop and the prop strut, making it maybe 5? 6? inches from the strut to the back of the prop.

If nobody turns up with an answer, try looking at some of the PHRF fleets in the midwest or northeast, you may be able to contact another I28 owner directly that way.
 
#10 ·
"I bought the boat with an electric motor,"
Could easily be read to mean you bought it with an electric--but are in the process of restoring the proper diesel engine. Since the usual prop would be relevant for the usual diesel engine, not the electric.

Either way, David Gerr has a "Propellor Handbook" out there. From the displacement of the boat, the engine speed, etc. there are formulas for figuring the correct prop to use. That would be another way to go, and to verify differences from the diesel setup. Although I'd figure, the same prop at the same prop speed should be used for the same boat, regardless of what was driving the prop.

A good prop maker might even know what they've sold for the boats before.
 
#13 ·
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
 
#14 · (Edited)
510,

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.

Tom
 
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