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Old 02-06-2017, 04:13 PM   #1
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Question Engine Size vs Hull Speed

I think I already know the answer to this but I'll ask anyway. The calculated hull speed for my 41 ft. full displacement trawler is 8 kts. Will (in theory) changing from a 120 hp to say a 200 HP (single engine) make much difference in speed. For now let's ignore changing props, etc. Just stick to HP vs. hull. Thanks
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Old 02-06-2017, 04:21 PM   #2
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In theory, no.
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Old 02-06-2017, 04:29 PM   #3
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In practise too, no.
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Old 02-06-2017, 05:43 PM   #4
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If it is a true full displacement hull, you might get a very small increase in full throttle speed and burn a tremendous amount of fuel for that small increase in speed.
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Old 02-06-2017, 06:00 PM   #5
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Depends on how the boat was powered.
A FD hull has a very narrow range of power needs. Usually 3 to 4hp per ton is fine. My Willard has 5. I'm just a tad bit overpowered at 5hp per ton w my Willard.

I,ve heard of a Litton but can't recall the hull form. Many here think they have a FD hull and actually have a SD. But if your Litton is a FD hulled boat 120hp is plenty of power. You could do well w 80 - 100hp. 150hp may get you another 1/2 a knot continous speed. 200hp may not get you another knot.

The real practical cruising speed for your boat is probably 7 knots. Working the engine harder (and that could be a good thing) may get hull speed and it may not. My boat running 500rpm down from WOT at a rated rpm of 3000 gets me to 1/2 a knot from hull speed. Hull speed on a FD boat is not something to "attain". It is to be avoided. Making much too big a wave and slight reductions in speed will increase economy quite a lot.

A FD boat will be considerably overdriven at HS. Seven knots will be your cruising speed and how much power it will take to make 7 knots will depend mostly on weight. How much boat are you pushing?
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Old 02-07-2017, 10:12 AM   #6
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The info that is stamped on the stringers by the engine says 11 ton and 15 ton gross.
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Old 02-07-2017, 10:25 AM   #7
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I have a 120 hp Lehman single. My speed at 1800 rpm is around 6.5 kts. Been thru the prop swapping routine. I'm now over propped; rpm top out at 2300 as compared to the specs for a 120 is 2500. But this prop gives me the best cruise speed at 1800. The other trawlers in our group cruise at an comfy 8kts. Makes taking a group trip real embarrassing. Not LOL!
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Old 02-07-2017, 10:52 AM   #8
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Dixie:

Over propping by 200 rpm (phototached, right!) while always cruising at 1,800 is not so bad, but not ideal. Unfortunately with your hull shape and condition if you propped correctly to reach 2,500 rpm at wot then to make your current 6.5 kts you would have to run at about 2,000 rpm. And to make 8 kts you would probably have to run near wot. You couldn't stand the noise and vibration!!!

So to keep up with the others and make 8 kts you probably would need 200 hp so you could cruise using about 100 hp.

The problem is that your hull is very inefficient or very heavy.

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Old 02-07-2017, 10:55 AM   #9
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Dixie,
Yup .. to make speed one needs power and the best power is at rated rpm.

What do you mean "the best cruise speed". I don't get it.
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Old 02-07-2017, 11:02 AM   #10
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A nice goal in spec'ing an engine is to pick one so cruise speed is about half of rated hp. This is often not possible on planing boats, but probably pretty good on a trawler.

Another goal is to get cruising speed rpm around the rpm where engine makes peak torque on the full load hp/torque/rpm curve. Around 1600 on a 120 Ford. Does not apply to planing boats, they need to run higher.

Very crude rules of thumb, but gets you close.

And it is no problem to overprop on a trawler, but with a caveat: You don't want to run it hard afterward. The last few hundred rpm of range are now a no-go zone.
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Old 02-07-2017, 11:19 AM   #11
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If true FD hull then it will only go so fast regardless of power. hull speed and all its variations are for calm water. Extra power does come in handy when facing large head seas that tend to stop or slow the boat. In that case the extra power gives better acceleration from the stalled condition. It may help in strong headwinds as well.

If you found a good eal on the larger engine I would not hesitate. Just because you have a bit extra power does not mean you must use it.

The only downside may be at resale time when some theorists will claim the boat only needs 13HP or such. []
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Old 02-07-2017, 11:47 AM   #12
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"The info that is stamped on the stringers by the engine says 11 ton and 15 ton gross."

The documentation numbers have ZERO to do with the boats weight , displacement.

The tonnage is a simple measure of volume so port fees could be collected.
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Old 02-07-2017, 11:51 AM   #13
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Well, the reason I started with,"I already know the answer", is I've been told by many that a FD hull will only go so fast regardless of HP. So, I guess 6-7 kts is it. I'd be glad to drop whatever engine is necessary if I could get 8, 9, 10 kts. but I don't think that will ever happen. But I'm always looking for new ideas.
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Old 02-07-2017, 12:25 PM   #14
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I have the Albin 40 (close to 34 feet at the waterline)...with a 120 Lehman.

Over the last 12,000 miles, I average 6.3 knots at around 1650-1700 rpm and burn about 1.9-2.1 gph.

Trying to get much above that and economy drops way off.

And mine is semi displacement.
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Old 02-07-2017, 12:38 PM   #15
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Quote:
Originally Posted by Dixie Life View Post
Well, the reason I started with,"I already know the answer", is I've been told by many that a FD hull will only go so fast regardless of HP. So, I guess 6-7 kts is it. I'd be glad to drop whatever engine is necessary if I could get 8, 9, 10 kts. but I don't think that will ever happen. But I'm always looking for new ideas.
If you are lusting for speed, you need a different boat. Sorry. Changing engines just to get better speed is something many owners have done with very little success....unless it is a true planing boat. Or unless you are very sure as to the hull shape of your semi displacement hull. LItton spec'ed that engine for a reason. It puts you right in the sweet spot of 1.0...
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Old 02-07-2017, 12:58 PM   #16
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Thanks for that info PSNEELD, I feel better now. And Barker, I agree, the people that built the boat know a lot more than I. And the final word is, I'm trawler 100 percent, regardless of speed. Comfort is where it's at for me and mine.

Thanks All
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Old 02-07-2017, 01:00 PM   #17
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Quote:
Originally Posted by Nomad Willy View Post
...
The real practical cruising speed for your boat is probably 7 knots. Working the engine harder (and that could be a good thing) may get hull speed and it may not. My boat running 500rpm down from WOT at a rated rpm of 3000 gets me to 1/2 a knot from hull speed. Hull speed on a FD boat is not something to "attain". It is to be avoided. Making much too big a wave and slight reductions in speed will increase economy quite a lot. ...
Dropping from my hull speed of 7.3 knots to 6.3 knots (my normal cruising speed), reduces fuel consumption rate by 50%.
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Old 02-07-2017, 01:04 PM   #18
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And the final word is, I'm trawler 100 percent, regardless of speed. Comfort is where it's at for me and mine.
Me too!!!!
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