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Old 01-10-2016, 05:19   #1
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Repower Islander 32

Hi All,

I've finally decided to replace the old Volvo MD7A in my 1977 Islander 32, "For Pete's Sake". I've found a good deal on a Yanmar 3GM30F and am ready to pull the trigger on it.

Although this engine will fit into my boat and it's actually some 100 pounds lighter than the Volvo it's replacing it's pretty much at the upper limit, power wise, for a 32 foot boat. The boat is in excess of 10,000 pounds and I do extensive cruising to northern Maine and Canada from my home base in Narragansett Bay, Rhode Island.

There's an Islander 32 in for sale in Southwest Harbor and it's got a 3GM30F in it so I know it's been done before. The specs show that it will fit quite nicely and it will probably sit on the existing bearers without modification.

Anyone have any experience with this size hp combination? I'm expecting to have to upsize my 3/4 inch shaft to at least 7/8 and probably 1 inch. Any recommendations for prop sizes would be helpful as well.

Looking for thoughts.
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Old 01-10-2016, 07:36   #2
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Re: Repower Islander 32

As long as it fits, the engine should be no problem. Have a 7/8" shaft on my 3GM30f which has worked but shaft is short with very limited unsupported length on full keel boat. Might think about going with a 1" shaft if for no other reason than zincs will be more readily available. 7/8" is a small diameter that the local chandler may not stock if it's mostly power boats in your area.

FWIW, prop is a 13x12 RH on 13,000# boat. Diameter somewhat limited by aperture.
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Old 01-10-2016, 08:10   #3
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Re: Repower Islander 32

Thanks for your response. I assume your prop to be a 3 blade, am I correct? My boat is just over 10,000 pounds and the gear is a 2.6:1 ratio. I have a fin keel and spade rudder but I'm concerned about the clearance from the prop to the skeg. I am currently swinging a small 2 blade 13 inch prop and I'm not sure I have the clearance to safely go much larger than 14 or so. It's interesting that you are using a 13 inch prop in a much heavier boat. Thanks for your response.
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Old 01-10-2016, 08:28   #4
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Re: Repower Islander 32

The 3GM30 HP is rated at high rpm. On a comparative basis is less HP with similar engines. That's why it's lighter. Also It would help to know what year the engine is. They had some trouble for a few years with pistons. I have experienced it on two 3GM30's and SailorChic on this forum has on one.
I guess I'm just saying, beware and try to find out why a "good" engine was removed ?
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Old 01-10-2016, 09:06   #5
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Re: Repower Islander 32

The problem that you are going to encounter is that you wont be able to get a large enough prop under the boat to absorb the horsepower of the new engine. This is aggravated by the 2.6:1 gear box ratio that the new engine has.

Lets look at the specifics:
Volvo MD7a........2600 RPM engine with a 1.91:1 gear ratio gives a shaft RPM of 1361 at WOT.
Yanmar 3GM.......3600 RPM engine with a 2.6:1 gear ratio gives a shaft RPM of 1384 at WOT.

Almost the same shaft RPM but twice the power available means that you will need a much bigger prop to absorb that power, but there isnt room for such a prop under the boat. The smallest prop that has half a chance would be a 16 inch 2 or 3 blade prop. Smaller than that and you will be driving the prop into cavitation at higher RPM.....which might be livable but a bit noisy.

So what are the options?
1. Forget the 3GM and look for something with less hp.....
2. Swap the 2.6:1 gear box for a 2.2:1....it wont be as bad
3. Go with the 3GM as is and fit the biggest prop possible and live with it
4. Rebuild the shaft line/angle to give more prop room

DougR
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Old 01-10-2016, 10:21   #6
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Re: Repower Islander 32

I repowered my CD30C that had the MD7A with a 20hp Beta from Oregon Marine Industries.
email address: omi@integra.net
The owner, Ben Thomas also provided pre drilled SS adaptive stringers. Ben provided a detailed drawing to align the motor mounts. The new engine fit perfectly.
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Old 01-10-2016, 11:23   #7
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Re: Repower Islander 32

I have a older 1972 islander 34 (12,000#, full keel, 26 foot water line) I think the 3gm30 is a tad oversized for my boat. I generally run 2100-2300 RPM. That's all it takes to get it to max. hull speed. If it's really choppy and winds 25 knots or so on the nose I'll do 2500-2600 RPM. Never needed more.

I suspect that a 2gm20 would work just fine in a 32-34 foot islander. Specially a Perry MarkII like yours.

It was the 1994 Yanmars that had a batch of bad pistons in them. You do NOT want a 1994 Yanmar GM engine. Also check the external oil line as they tend to rust through 3" in front of the started. It should be smooth tubing, if it looks rough at all, replace it. Yanmar has a copper replacement line for it.
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Old 01-10-2016, 11:38   #8
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Re: Repower Islander 32

Quote:
Originally Posted by spflyer View Post
"For Pete's Sake".
Why for my sake?

We have a similar sized boat that despite sailing in NW Europe with big tides, has an engine that is over sized. I suspect only because Moody were offered a really good deal on a large number of Volvo 2003 engines. They seemed to have used the same engine in everything from 31 to 34 feet in length. In use we never go beyond 2600 revs which is probably 20hp out of a possible 28hp when new.

If or when I re-power I will be looking at 20-25hp to save weight and cost. The newer model to ours has 20hp.

We have a long shaft about 4ft so its 1" in diameter and swings a 16x14 2 blade prop with a 1:2.3 box. Anodes and dripless seals are readily available for this size range and the shaft was cheap enough when replaced a few years ago.

The point why is it for sale? is worth following up though.

Pete
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Old 08-10-2016, 09:07   #9
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Re: Repower Islander 32

Yeah, I've been doing a lot of research on this and you're absolutely right about the prop thing. I've only got 3/4 inch clearance from the existing 13 inch prop to the hull and that's not enough even for the existing prop. I'm shifting my search to a more appropriately sized engine, probably a Yanmar 2GMF or a 2GM20F. They're rated at a little less RPM and will probably accommodate a 13 inch prop. A Beta 16 would be my first choice but it's just not in the budget. The MD7A actually moves the boat fast enough in flat water/light wind but suffers when either kicks up.
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Old 08-10-2016, 09:26   #10
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Re: Repower Islander 32

Many boats are over powered. It's not a bad thing necessarily. Just get the best engine you can and don't go too low on HP. It only takes a few HP to make hull speed on a 32 footer really, but most installers put in bigger engines for durability and more HP in a pinch.... or just plain due to the engine sizes offered. My 30 footer had 25 HP. My 31 footer had 18 hp. The 31 would get a little too hot whenever it was pushed. The 30 footer you couldn't get it hot if you wanted to. My 44 footer had 51 HP. My 47 footer had 85 HP. The 44 was far more economical and would motor just as fast as the 47. An 8 hp outboard will easily push a 30 foot powerboat to displacement hull speed in flat water.
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Old 08-10-2016, 09:53   #11
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Re: Repower Islander 32

Quote:
Originally Posted by spflyer View Post
Yeah, I've been doing a lot of research on this and you're absolutely right about the prop thing. I've only got 3/4 inch clearance from the existing 13 inch prop to the hull and that's not enough even for the existing prop. I'm shifting my search to a more appropriately sized engine, probably a Yanmar 2GMF or a 2GM20F. They're rated at a little less RPM and will probably accommodate a 13 inch prop. A Beta 16 would be my first choice but it's just not in the budget. The MD7A actually moves the boat fast enough in flat water/light wind but suffers when either kicks up.
It's not a huge deal, just stay within spec and pitch it correctly. My 300 HP powerboat had a 16" prop. (yeah some different things going on there) But you live with what you have. Larger diameter is theoretically more efficient, but realistically, you don't want to run a 3600 rpm diesel at 1200 rpm even if you could swing a 20" prop!
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Old 08-10-2016, 11:08   #12
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Re: Repower Islander 32

You can of course talk to a few prop experts to get professional advice on what'll likely work best. And if you have the room, since you're swapping out shafts & thus brackets, you might look at using a shaft which is a bit longer in order to accomodate a bigger prop. It would just mean moving the bracket a bit further aft when you change it, assuming that doing so would give you significantly more room to swing a bigger one, without it being to close too the rudder or anything else.

Also, you're probably already well aware of this, but use caution in terms of how much you spend in repowering the boat. As it'll be difficult at best to get the funds back out of her when you sell her.
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Old 08-10-2016, 11:28   #13
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Re: Repower Islander 32

Here is the formula for shaft diameter. http://www.pacificmarine.net/enginee...ller-shaft.htm Someone should check my numbers. I used a safety factor of 5 because engine is a diesel. The horsepower was 27 and the propeller shaft speed is 1384 RPM with an engine speed of 3600 RPM with a 2.6 reduction in gear box. For Aqualoy 22, the diameter is 0.630 inches and for Monel 500 0.6348. These alloys will not result in crevice corrosion which will happen with 314 stainless. The engine HP of 27 is for only short duration of one hour. The maximum continuous HP for the Yanmar 3GM30F should be at 3400 RPM with the same propeller and gearbox.
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Old 08-10-2016, 12:02   #14
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Re: Repower Islander 32

The horsepower recommended by Ted Brewer in his Understanding Boat Design on page 58 is 3 to 4 horsepower per ton is adequate and 5 is ample. Note that tons are long tons and are 2240 pounds per ton. The Islander 32 weights 10500 pounds empty, but will weight more with provisions and crew. Looks like 27 HP is more than an ample amount.
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