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Old 20-12-2015, 10:03   #46
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Re: Engine size for boat size

@ WayneB #38

Thanx Wayne - you are absolutely right!

I went pack to my tiny print-out of the power curve. 1 1/4 x 1 1/2 inches, read through a magnifying glass. A more careful reading sez just under 8 HP at 1,800. My fault for not applying the formula which I actually do know :-).

Thanx again

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Old 20-12-2015, 11:09   #47
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Re: Engine size for boat size

Good Thread.


I am just finishing a repower on my 90K displacement boat and had to go through all the variables and calculations mentioned above. Interesting that Gord May nails my configuration with the 1HP to 500Lbs rule of thumb. I looked at a lot of scenario's: engines, transmissions and various prop configurations. I went with the Cummins 6BT 5.9 210 reman to avoid the common rail engines. I'll be doing start-up sometime next week and that should tell me if my selection was accurate. One factor to be aware of on a repower is shaft diameter. If you over size the engine, it may exceed the shaft integrity and twist it off. I just backed into the selection with displacement, LWL, transmission ratio and prop size. I did have to increase the exhaust size by an inch for the Turbo (3" to 4") and consider many opinions saying I needed a 6" exhaust really disturbed me. But on a displacement boat and with the engine under the waterline, I decided the 4" could handle it since I selected a 210 Hp and only need 180 Hp when not running the Engine drive refrigeration unit and house bank alternator.
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Old 20-12-2015, 11:17   #48
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Re: Engine size for boat size

Quote:
Originally Posted by FlightPlan View Post
Good Thread.


I am just finishing a repower on my 90K displacement boat and had to go through all the variables and calculations mentioned above. Interesting that Gord May nails my configuration with the 1HP to 500Lbs rule of thumb. I looked at a lot of scenario's: engines, transmissions and various prop configurations. I went with the Cummins 6BT 5.9 210 reman to avoid the common rail engines. I'll be doing start-up sometime next week and that should tell me if my selection was accurate. One factor to be aware of on a repower is shaft diameter. If you over size the engine, it may exceed the shaft integrity and twist it off. I just backed into the selection with displacement, LWL, transmission ratio and prop size. I did have to increase the exhaust size by an inch for the Turbo (3" to 4") and consider many opinions saying I needed a 6" exhaust really disturbed me. But on a displacement boat and with the engine under the waterline, I decided the 4" could handle it since I selected a 210 Hp and only need 180 Hp when not running the Engine drive refrigeration unit and house bank alternator.
===

A couple of questions: I'm curious to know why you wanted to avoid common rail engines? Also, when you say that you only need 180 hp, is that the calculated power to reach hull speed? If so, I think I'd want a little extra reserve, especially if you also need to drive a refrigeration compressor and high output alternator.
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Old 20-12-2015, 12:26   #49
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Re: Engine size for boat size

Quote:
Originally Posted by FlightPlan View Post
Good Thread.


I am just finishing a repower on my 90K displacement boat and had to go through all the variables and calculations mentioned above. Interesting that Gord May nails my configuration with the 1HP to 500Lbs rule of thumb. I looked at a lot of scenario's: engines, transmissions and various prop configurations. I went with the Cummins 6BT 5.9 210 reman to avoid the common rail engines. I'll be doing start-up sometime next week and that should tell me if my selection was accurate. One factor to be aware of on a repower is shaft diameter. If you over size the engine, it may exceed the shaft integrity and twist it off. I just backed into the selection with displacement, LWL, transmission ratio and prop size. I did have to increase the exhaust size by an inch for the Turbo (3" to 4") and consider many opinions saying I needed a 6" exhaust really disturbed me. But on a displacement boat and with the engine under the waterline, I decided the 4" could handle it since I selected a 210 Hp and only need 180 Hp when not running the Engine drive refrigeration unit and house bank alternator.
What kind of prop do you have?

On a 90k displacement boat, I would want a Hundested for sure. A fixed prop in that size will be like pulling a sea anchor behind you.
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Old 20-12-2015, 13:36   #50
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Re: Engine size for boat size

One more observation.

We have a 17 ton boat with an old 96 HP truck engine that has been propped in a way that it produces no more than 65 HP. The logic of this confused me initially but now I see that the engine is very very happy at around 1200 rpm at which we are getting 6 knots and great economy with plenty of room to throttle up to 2000 rpm if we run into trouble. We won't go any faster for all the extra rpm but we can heave ourselves out of trouble. The downside is that the engine is pretty heavy, at around 420 kg including the transmission. At least it is down low in the boat so contributes to our stability.


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Old 20-12-2015, 14:42   #51
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Re: Engine size for boat size

I have a larger engine then most a Perkins 4.2, , 85 HP in a 39 ft 12 ton sailboat. Couple of thing s to reiterate is weight will increase if cruising offshore or for extended periods, and its always nice to have that little umph when you really need it. Bigger is better I think , to a point.
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Old 20-12-2015, 15:06   #52
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Re: Engine size for boat size

@ Dockhead #49

Is it as simple as all that :-)?

Here is a neat link to some things Hundested:
http://www.svseeker.com/controllable..._propeller.htm

Hundested props are the cat's whiskers, but they require a Pitch Control Unit (CPU) that sits where the reduction box would normally sit. Modern ones are hydraulically controlled, of course, but it usta be manual: A handwheel of about 15" diameter with a crank handle on the rim. Monel - lovely to look at and felt wonderful in the hand. This wheel was shafted to the PCU via as many universal joints as it took.

I have a hunch that the PCU for even the smallest available Hundested prop will cost more than a reduction box, not to mention the cost of the prop itself.

In my callow youth the Hundested c/w PCU was driven by a semi-diesel that would idle happily at 60 RPM and topped out at something less than 400! No reduction box necessary.

The "hot head" engines were of many makes, the SABB being a common one. Less common were the Hundesteds but here is a link to the start-up of a 45 horse Hundested. They required a certain "touch".



AFAIK the smallest prop made by Hundested these days is meant for about 150 horses. Since in a yot it is unlikely that anyone would carry the weight and sheer size of anything other than a modern high-speed diesel, I would think that a reduction box would be necessary in addition to the PCU.

I wouldn't think that a cost/benefit analysis would speak in favour, but please do educate me :-)

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Old 20-12-2015, 15:11   #53
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Re: Engine size for boat size

My 45t (90,000#) Ketch is adequately powered with a 135hp Lehman. 3-1 reduction & (the secret) a 30" Hundedsted CCP
Can hit 9.5 at WOT, cruise at 8.0 burning 2.5 gph



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Old 20-12-2015, 20:16   #54
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Re: Engine size for boat size

Quote:
Originally Posted by wayne.b View Post
===

A couple of questions: I'm curious to know why you wanted to avoid common rail engines? Also, when you say that you only need 180 hp, is that the calculated power to reach hull speed? If so, I think I'd want a little extra reserve, especially if you also need to drive a refrigeration compressor and high output alternator.
I have a lot of limitations in draft both above and below the waterline which leaves me in the bluewater class of sailing. Common rail engines are run by computers mostly and the software is proprietary within the manufacturers specifications. The software can be bootlegged, I know that, but I don't like that option. But back to the subject: In many cases when a fault is recognized by the computer, the system will shut down and it requires a certified factory mechanic with the software to clear the fault. I chose not to deal with that issue. I wanted something that I might better service myself with selected spare parts on board when offshore, and not limp into passes and ports at 3 knot until the fault could be cleared.

I agree on the extra HP for the compressor and alternator. The calculations came up to 180 HP for the boat and drive train, but the extra effort is covered with the 210 HP that I am installing. The compressor only needs to run about 90 minutes every 12 hours to drive down the temperatures in the Freezer and Refrigerator. The alternator is also a variable that I cannot calculate accurately until I record the data. But I think I have enough to make hull speed with all systems running.

In a flat water situation and all systems charged, I am overpowered by 30 HP, but making 9.2 knots theoretic hull speed. That calculates to 2600 RPM on a 2800 RPM engine.
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Old 20-12-2015, 20:26   #55
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Re: Engine size for boat size

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What kind of prop do you have?

On a 90k displacement boat, I would want a Hundested for sure. A fixed prop in that size will be like pulling a sea anchor behind you.
I have a 24" three blade Max-Prop. I have wanted to get a 5 blade Max-Prop, but the retail price from Max-Prop, got jumped up 25% when quoted to me from the distributer. I always thought retail meant the distributer was covered since it was coming direct from the factory. (no handling in other words) One of those deals I just didn't do on a principle basis.
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