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Old 21-11-2010, 21:15   #1
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40' Jim Brown Searunner

Ok I've got a 40 Sea Runner and I'm going to reposed it with a Diesel I expect a 30 horse. If I'm running a 16" two blade adjustable feathering prop what gear ratio should I be using I've had two opinions. 2 to 1 or 2.6 to 1. What do you think and why would you chose that option.
By the way I have a 28 hp Palmer gas engine with trany
available for any of you that are interested.
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Old 22-11-2010, 07:49   #2
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Generally, if installed in a small, planing boat, the 2/1 would be better.
Searunner is not going to plane,so the 2.6/1 will yeild a slower shaft speed, which will be quieter and make less wear. 1000 rpm shaft speed is very civilized. New diesels turn faster. Large dia. ,slow turning wheel is better for ( non planing) motoring, but more drag for sailing, but yours' feathers !
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Old 22-11-2010, 08:56   #3
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Select the ratio that gives the optimum rpm of the engine to prevent over revving or lugging. The engine will appreciate it over the long haul.
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Old 22-11-2010, 20:54   #4
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I repowered my 37' Searunner with a 2GM 27 HP Yanmar, and I took the reduction gear they provided. Then the only variable was the prop. We got lucky and hit it right the first time.
There is a lot of Searunner guys over at the forum. It has been going strong for two years, c'mon over.

Trimaran - Especially Searunner - Owners
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Old 02-11-2011, 14:07   #5
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Re: 40' Jim Brown Sea Runner

Mr Molan,

I have a 37 ft Searunner with a 27 hp Yanmar here on St THomas. What size prop did you have on your boat?

I have a 13-inch, fixed 3 blade prop on a 1-inch shaft and the boat will not do over 5 knots. Looking for advice on how to re-prop.

Many thanks,
Bob Petersen
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Old 10-11-2011, 08:47   #6
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Re: 40' Jim Brown Searunner

I have a Searunner 40 with a Yanmar 3GM30F engine. I chose the lower shaft speed ratio, at the recommendation of the dealer. I swing an 18" diameter (and I recall a 12" pitch) Martec Mark III folding prop, on a 1" shaft, 17' long, supported every 40" by bearings. My cruising RPM is about 1800 and results in a vibration-free hullspeed of a bit more than 8 knots in flat water. The prop tip is two fingers clearance to the hull.
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Old 10-11-2011, 09:33   #7
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Re: 40' Jim Brown Searunner

I screwed up and didn't make the time limit for edits, so here's the correction: The prop is a Martec Mark III Elliptical, RH 16" diameter by 10" pitch. Cruise RPM is 2200 resulting in a fuel consumption of just over half a gallon per hour in flat water. I was going to send pics, but they hardly seem necessary.
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Old 13-11-2011, 09:49   #8
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Re: 40' Jim Brown Searunner

Not quite comparable, but this is what I am planning to install in my CC44 Marples tri. 18" Martec elliptical prop with 14 pitch being driven by a M50 Universal diesel 44 HP. with a 1.9/1 HBW tranny on a 1 1/8" shaft . Some experts think the match is "pretty close" but who knows. The hull isn't in the water yet, and I have to put something in. Any thoughts on my expected install?
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Old 13-11-2011, 18:26   #9
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Re: 40' Jim Brown Searunner

Hope the shaft is pretty short if only 1 1/8" Also the 1.9:1 seems less than ideal. 2.5:1 would seem more likely.

Good luck.

Jeff
soon to be CC 35 owner???
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Old 03-12-2011, 09:15   #10
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Re: 40' Jim Brown Searunner

John, All engine/prop combinations are compromises. The 2.6 reduction will need a larger shaft and prop (heavier+ more$) because it produces more torque at the prop. It gives more bite too. But the 2.0 is what I recommend, along with a 3 blade feathering prop - to two blade feathering props are known to rattle but the three blade is smoother. And it will cost lots less and since the diameter is smaller, it will be not as deep. The difference in operation is almost nil in flat water, but the smaller prop will be slower in waves and a breeze.
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Old 03-12-2011, 10:03   #11
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Re: 40' Jim Brown Searunner

John (Marples), I'm a little confused. Are you recommending a 2.0 reduction and a smaller shaft than 1 inch? I throw a 16 X 10 two blade on a one inch shaft, and get performance in going to weather in chop, with little vibration. This may be due to the bearings every 40", and good alignment. Why would anyone choose a lower performance prop that operates best only in flat water? I could understand using a Gori three blade folder, but it would only give me better thrust in reverse, as I achieve hull speed at cruising RPMs, which is an efficient use of the Yamnar 3GM30. But, the Gori is a lot of money, and I already have good performance with the Martec. Am I missing something in this discussion?
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Old 03-12-2011, 11:54   #12
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Re: 40' Jim Brown Searunner

Roy, It sounds like you have the right combination of prop and engine but the shaft diameter is marginal for 27 HP. It should be at least 1 1/8. The problem is the propeller is cantilevered out much farther than a fixed prop and any off-center loads can break the shaft. The martecs tend to be a little rattlely and that vibration can cause stress fatigue.
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Old 08-12-2011, 08:09   #13
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Re: 40' Jim Brown Searunner

John M to continue my original comment regarding my 40' Sea Runner. I've since installed a out of the box new 24 year old M30 Perkins which found for a good price. Hurth 2/1 gearbox. So I will go will that and I have the original prop which is 16" feathering two blade on a 1" shaft. I still need to adjust the pitch of the prop as it won't rev past 2000 but that will be done during haulout this winter.
The Perkins runs as expected excellently although the tach isn't always (anyone have tach?)
I've had the Searunner only 2 years now and I'm still in the middle of it's rebirth as it was a 1975 vintage, still a strong boat! El Condor is the name Jim's sailed on it and we're part of the Toronto Multihull Sailing Cruising Club which I've been a member of for 22 years. I expect that the refurbishment will go on for one more season and then our retirement will commence at least in some form looking at the economy.
Any coments? Oh I could really use some support with posting photos.
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Old 08-12-2011, 10:11   #14
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Re: 40' Jim Brown Searunner

I discarded my engine panel and fabricated a new one to include both analog instruments and "idiot" lights with piezoelectric buzzers (different sounds for different alarms).

I also installed external switches for all cockpit related lighting needs.
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Old 08-12-2011, 12:35   #15
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Re: 40' Jim Brown Searunner

Ok Ok I 'fingered' it out.
This is three veiws of the Feathering prop and a picture of the old beast.
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