Islander 28

[ISLANDER28SCOTT]

I just acquired a 1984 islander 28 and would like to ( I Think ) change the 2 blade prop for a three blade. It has a 2gm 13 hp Yanmar. We are going to be running down the Tenn-tom in the fall and up it in the spring against the currant. The two blade will get the boat up to hull speed, but will it be enough going back to Kentucky lake in the spring from the Keys? Props are pricey and I don't want to waste cash.

[hobopacket]

I am not sure a prop is going to suffice. If the prop you have now gets to hull speed. That's all you got is hull speed. Just hope the current isn't running faster or as close to it.

Basically what I am saying is a new prop might only get you .25 knots. Worth it?

[GordMay]

Greetings and welcome aboard the CF, Scott.

According to Cliff Friesen, of West by North Enterprises (makers of “Campbell Sailer” propellers)
➥ http://www.westbynorth.com/choosing-a-propeller/
➥ http://www.westbynorth.com/campbell-sailer/

Two Blade Fixed Blade Propellers

This old belief is the main reason not to go with a two-blade propeller. Two-blade propellers do not belong on a sailboat. (Very personal opinion) Disturbed water is the main reason. Every keel has disturbed water coming off of it, be it a streamlined fin keel or a full keel. Water meeting on the trailing edge takes time for the flow to smooth out. Two-bladed propellers turning, go through the disturbed water at the same time. This causes a slippage, in turn inefficiencies because it is on/off/on/off etc.

How many skippers with two blade propellers notice the stovetop, lifelines or something else humming, at different engine rpm? How many blamed the engine? This is usually caused by the harmonics set up by the two-blade propeller. Sometimes in combination with the harmonics of the engine, it can cause even more uncomfortable zones in engine rpm.

You also get turbulence created by the over sized propeller. It is oversized to compensate for its lack of efficiency, usually in pitch. The pitch directs water to the other side of the keel or propeller. This happens at the top and bottom blade when vertical behind the keel or strut. This causes turbulence and thus drag with the exception of the very small inefficient propellers.

Three Blade Fixed Propellers

A three blade propeller is the better way to go. You only have one blade moving through the disturbed water at any time. Two blades are still pushing the boat. We have more efficiency, more thrust, more control and less vibration. Depending on the propeller mind you it may cause slightly more drag. For some propellers it is the equivalent of dragging a hand through the water. Most sailboat builders use, as the standard fare, heavens forbid, a powerboat propeller and the drag can be much worse, like dragging a bucket. At best, in order to cut drag, some propeller manufacturers cut down a powerboat propeller to make it have less blade area, loosing the majority of its thrust and ability to control the boat. To compensate, they increase its size either in diameter or pitch. It is quite common to have more drag with a standard two-blade powerboat style propeller than a properly designed sailboat three-blade propeller. A properly designed sail boat propeller will give the vessel good thrust in forward and reverse from minimal horsepower engines and offer low drag when sailing. There are patented sailboat propellers with low blade area (read less drag) that have, through design, maintained and exceeded thrust figures compared to standard style propellers. An added benefit or disadvantage is that they can often reduce the prop walk in reverse. Most skippers like to be able to back in a straight line. Many skippers like the simplicity of a fixed propeller along with its lower price.

[appick]

I agree with Hobo, a 3 blade prop would get you more immediate transfer of power from the boat to the water, good for when you're motoring into waves. With a 2 blade it would take the boat longer to get back up to speed in this scenario. What it won't do is make you go faster. Water line length, vessel weight, prop diameter, pitch, horsepower, and torque all play into the game here. I won't pretend to have a comprehensive knowledge of it I just think I know enough to be dangerous! If you give the people at Michigan Wheel a email or a call I'm sure they'd be willing to share their knowledge and not lead you astray.

There are many prop shop's or brands that have online calculators too, if you email these folks they usually respond. Michigan Wheel is a Mfg that makes a lot of props and hence has people with a good understanding of what is what. In my search I've had many tell me they couldn't offer me anything to help me get what I wanted (more top end speed, I need more HP). I guess at least most of the one's I've emailed or talked to figured it's better not to sell me a prop, than to sell me one and have me be disappointed.

[Pete7]

If you have a cheap way of lifting the boat enabling you to change props then it would be worth using the calculators and finding the right size of 3 bladed prop you need, note check to see if you need right or left handed! Then off to e bay or Craigslist etc but you have to be patient. If the cost of a lift is expensive then might not be worth the effort because you might not get it right the first time, so the cost goes up.

Last spring I picked up an identical propellor for £25 on e bay, but it took 2 years to find. It means that if something happens like hitting a submerged object I now have a spare prop available. Also I can antifoul it at home ready to go rather than faffing around in the mud or a cold boat yard, hoping the paint will dry.

You are planning quite a bit of motoring so yes a 3 bladed prop would be nice and keeping the existing prop as a spare or use if you decide to do a lot of sailing.

Pete

[ISLANDER28SCOTT]

Thanks for the input. I understand hull speed is hull speed and trying to get more than that is fruitless. Its the slip that seems so excessive. I back out of the slip turning the boat then shift forward and she keeps going backwards for a boat length. Finely stops then sets there for a short period of time and slowly starts to creep forward. When heading into the wind a wave on the lake will slow her down a bit, maybe a knot, then it takes her awhile to recover. Its this grip on the water that has me thinking of a proper sized 3 blade and the river systems we will traveling. Maybe I'm expecting to much out of 13 hp pushing 9,000 lbs. of boat.

[DougR]

In general, a two bladed propeller is more efficient than a three (or more) bladed propeller, and this is because the more blades you add, the more inter-blade interference occurs. However, this assumes there is adequate room under the boat to fit a two bladed prop with enough blade area to absorb all the horsepower that the engine can deliver. Unfortunately frequently the vessel design precludes such a large diameter propeller, and in these cases a three blade prop frequently provides more thrust than a limited diameter two blade.

If you provide some info I can crunch the numbers and see whether a two or three blade prop looks best for your situation. I need:

- engine h.p.
- transmission reduction ratio
- max. rated engine RPM
- current prop diameter
- approx. current prop tip clearance ( prop blade tip to hull distance)
- current flat water max speed

DougR