How to choose a suitable engine

[orang laut]

Hello folks,

I'm coming in here with some questions regarding choosing a suitable engine. Have a basic clue that there has to be some sort of correlation between the amount of hp with the size/displacement/purpose of a boat; one doesn't plonk in a 115hp Ford to move a 15 tonne cruiser... this much I'm sure of.

Is there a basic calculation?

Boatbuilding plans are in progress, and would be helpful to be able to narrow down the cost of obtaining, installing, maintaining & all the other
-ings beforehand.

The engine is intended for a wooden sailboat that's about 37' on the waterline, my (uninformed) guess is that it would weight around 12 to 15 tonnes all in.

Would be helpful if explainations could be given in chick-friendly lingo.

Thanks in advance

[FAST FRED]

3 hp per ton will do fine , with NO reason to go over 5hp/ton .

The tons you list may be VOLUME (for extracting port tax) .

The actual displacement is required to chose power.

A 35 or so hp inboard would be fine , but a bit more would be required if a very large alternator (150-300A) or refrigeration is driven from the main engine.Much over 60hp would shorten the service life from underloading.
\
For realistic service life the engine should have 2 cubic inches of displacement for each cruise HP. The engine displacement will be listed by the mfg.

a Liter is 61 cu in if the builder is euro.

FAST FRED

[orang laut]

Ah yes, just realised my mistake there with the tonnes vs tons. And right on about the volume... this is what's going to be written on the boat papers for tax/registration purposes.

How do you calculate the actual hull displacement? I was told that this is the "Block co-efficient" calculation, yet can't seem to find any examples over the internet.

Actually boat's planned to be built in Indonesia, so pretty useless to confer to them for advice.... seen 20-something hp made-in-china 'DongFeng' (or something like that) engine squealing hard to move a timber boat that's almost 17m on deck. Nuts.

Thank you Fast Fred.

[Alik]

If You are in Malaysia, probably You read 'S.E.A.Yachting' magazine? I published a simple speed/length/displacement/power diagramm in Jan/Feb 2006.

[FAST FRED]

"I published a simple speed/length/displacement/power diagramm in Jan/Feb 2006."

If it would not violate the copy right of the magazine , would you please post it here? Many could use this information.

FAST FRED

[Frosty]

All these formulea and advise is ok if your going to buy a new one. I would think that you will not therefore your choice is limited to availability.

personally I dont think your theoretical 15 tonner with 155hp ford is so ridiculous. Here at the RLYC my friend has a 36 foot Nauticat --wieging in at 8 tons say. He has a 100hp Volvo, nice when the wind is on your nose.

You will not save money on fuel by undersizing, A diesel use 5gallons per hour per 100hp used. So your 100hp will use 2.5 gall per hour if you are absorbing 50 of those HP, but you have a 100 if you need it.

Personally in these modern days of high power compact diesels theres no excuse for being under powered.

Sorry to disagree with Fast fred,-- sorry Fred but I would be starting at 60hp up. 30% is sufficient load to stop cylinder glazing. ( if glazing even exists in todays engines and oils)

Theres many threads here on people wanting more Hp but cant find the space, --its too late shouldnt have built the boat around the engine. I have never heard people say I have too much Hp. You will not be disappointed with too much but the boat could be a dog with not enough.

[Alik]

2 jack frost
You are right. Before it was considered sufficient to have 2-4 h.p. per ton of displacement, but today's tendency is to have up to 6 h.p. per ton and even more for multihulls.

[Alik]

This is the diagram, developed by me using calculations by Delft Series.
N/DISPL - power (kW) per displacement (metric tons)
Fr=Fn Froude number
LWL (m) - length of waterline
Line 1 - ORC SR minimum speed under power
Line 2 and 3 - lower and upper limits for most of boats
This diagram works for monohull boats, with fixed and feathering propellers. For folding props, increase power by 20-30%.

[Guillermo]

Nice diagram, Albert. As per a quick check, it seems to me to gave the DIN bhp for long range cruising, so around 80% of the continuous power of the engine. To get the DIN rated maximum power you'd need to divide the figures got from the chart by 0.72 or the like. Is this so? Could you clarify what should be understood as the power given by the chart?

I agree with you about power nowadays: It uses to be in the range of 5-6 HP/tonne (continuous rating) or even higher. In my opinion 5 HP/tonne is pretty good to provide the extra muscle needed in some occasions, but not being as high as to quickly deteriorate the engine because of the excessive underloading, as Fred correctly points out.

orang laut: difference between tons (2240 lbs) or tonnes (2204 lbs) is very small and for rough numbers and estimatives you may use either one or the other.
Cheers.

[Alik]

2 Guillermo
This diagam is in kW, not h.p.
Of course this depends on propeller settings and performance curve of the engine, but power on the diagram is generally actual power developed by engine. So if one needs boat speed for contionous RPM, just find out power corresponding to this RPM from engine data and then estimate cruising speed form diagram.

I use this diagram for 8 years already and it showed quite good matching to existing boats. Please note that this diagram is for estimation only.

[Guillermo]

Originally Posted by Alik

This diagam is in kW, not h.p.

Yes, of course. To compare I did the conversion to HP.

Originally Posted by Alik

...power on the diagram is generally actual power developed by engine. So if one needs boat speed for continuous RPM, just find out power corresponding to this RPM from engine data and then estimate cruising speed from diagram.

Thanks for the clarification.

Cheers.

[FAST FRED]

"30% is sufficient load to stop cylinder glazing. ( if glazing even exists in todays engines and oils)"

Cylinder glazing (actually the cylinder is burnished by low pressure rubbing) still exists in every underloaded diesel.

A few other under load problems are the oil gets contaminated rapidly from poorly sealing rings.The rings seal from combustion pressure BEHIND them , and with low loads create low combustion pressure and sealing.

The turbo may not be provided with enough exhaust to spin at needed pressure , so the engine becomes very inefficent , dumping more fuel in the cylinder than there is air to burn it.More oil contamination.

This can show as "Wet Stacking" where the exhaust is White from low compression , and oil (unburned fuel) builds up inside the exhaust system. Fancy electronic fuel management helps a bit , till the lightning strike.

Gas engines are superb at low load operation , once a diesel gets below 50% its nervous time in terms of service life..Diesels should be selected to work HARD! , and never oversized.

FAST FRED

[orang laut]

Thanks a million zillion... forgive me for keeping silent while the discussion rages on, I've got to buck up on all the terms used and imagination working double time to visualize what's described.

One small question: how is the hull displacement calculated?

Or is this substance for another thread?

[Guillermo]

Originally Posted by orang laut

One small question: how is the hull displacement calculated?

You need to know waterline length, waterline beam, and body draught amidships (just body, not total draught under the keel). Then you need the "Block coefficient" which may vary a lot depending on hull forms. But for a rough estimative on a sail boat it should be not far away from 0.4
So if you multiply Lwl*Bwl*Hd*0.4 you get (roughly) the underwater volume of the hull. Now multiplying this by the weight density of water (If salt then it's around 1.025 tonnes/m3 or 64 lbs/ft3 depending on units chosen) you get the displacement weight in tonnes (if you chose measurements in metres) or pounds (if you did it in feet).
As said, this is a rough figure just to get a first idea. Being accurate takes quite a bit of more time and knowledge.

Cheers.

[Jimbo1490]

Cylinder glazing (actually the cylinder is burnished by low pressure rubbing) still exists in every underloaded diesel.

This is really dependent on the type of rings and the corresponding final hone finish selected by the manufacturer of the engine. These days it is quite common to select rings and finish that require NO break-in period at all. So there is no danger of underloading the rings during the break-in period and ruining the cylinder wall finish. The engine is 'good to go' right out of the box. Of course you should check with your manual to be sure of the rings and any break-in proceedure, if there is one.

Jimbo