Engine Sizing and Fuel Consumption

[CarinaPDX]

I think by now it is clear that the answer to the original question is to not focus too much on the published fuel specs and instead choose the right size of engine for the boat and your type of cruising. Dashew's Cruising Encyclopedia had a formula for calculating the engine size - you might take a look at that. I can't find it at the moment...

Having just bought the new engine not too long ago, I have looked at what is available. I too really liked the Beta engines - the marine conversion is first rate. Most of the engines available today are conversions based on utility diesels; the only problem I have with that is that they don't fit in a boat as well because the flywheels are aft of the engine instead of in front. OTOH the new engines are smaller and lighter for the same power. My short list had Beta, Volvo, and Yanmar on it and I am convinced that they are all high quality. I dropped the Beta from the list simply because the marine parts could be a problem getting into many places - that is a problem with customs, tariffs and bureaucracy, not the vendor. There are few cruising spots that don't have Volvo and Yanmar dealers, so parts tend to be readily available (at a price).

[Jim Cate]

""There are few cruising spots that don't have Volvo and Yanmar dealers, so parts tend to be readily available (at a price).''

Ho, Ho, Ho! That's a really funny statement, mate! There are HEAPS of cruising spots without a sign of a Volvo or Yannie dealer, and in the more out of the way places where they do exist their stock of parts is often pretty slim.

IIRC, the Beta engines are marinized Kubota industrial/tractor engines. The major engine parts are available from the tractor dealers (often at lower prices), but of course, the marinization bits must usually come from Beta.
There is a similar situation with our Nanni/Kubota engine, but so far all is well (touch wood).

Cheers,

Jim

[Tony B]

Here is my take on the whole thing.....

Your boat weight = 9500 lbs. With fuel, water, supplies and humans lets say 11,500 lbs.
It takes 2 to 3 HP per long ton to move the boat not quite to hull speed.

A long ton = 2,240 lbs.

Divide 11,500 lbs by 2240 = 5.134 long tons.

Lets go with the 3 HP per ton deal so 3 X 5.134 = 15.4 HP.

So, 15.4 HP is needed to push yr boat to almost hull speed.

Normally, at this HP range requirement, the HP needed (15.4) is doubled to say roughly 30 HP. The 30 HP would probably be your most efficient engine because by the time you take away HP lost through actual heat from the engine, prop shaft loss and the prop itself you will be needing about slightly more than half to 2/3 of the 30 HP motor. With a little to spare for head winds, seas, etc.
A diesel produces approx. 18 HP per gallon of fuel. Given this, I would say that you can run at almost hull spread at 1/GPH.
If your boat is 32 Ft overall, your waterline is approx 27'.
The Square root of 27 = 5.2 ft.
Take the Sq. Root of your waterline (5.2') and multiply by 1.34 (this is a constant number) = 6.9 Kts for your theoretical hull speed. From this, we get 6.9 Nautical Miles/ Gal. To convert NM' to statute miles multiply by 1.14. So now we have 7.8 miles/gal. fuel consumption with a 30 HP.

[europaflyer]

Big thanks for all the replies everyone.

Having read through them all, I'm pretty much back with what I always thought for engine sizing - 2hp per 1000lb and round up. That would leave me with the 20hp, which is actually what the new boats come with now. The only reply really against the trend was Tony B, I respect your opinion but think it might be a little skewed seeing a powerboat as your picture? Also, the waterline is only 24ft which makes a difference.

As for the parts thing with Betas which came up, I've always been told that that's one of the strengths of the engine - the major components are available from tractor dealers the world over, and the marine bits added by Beta are what you would carry anyway if you are serious about spares. Whether or not this holds true, I will find out!

Again, I'm really grateful for everyone taking their time to help me with my decision.

[Cheechako]

Originally Posted by David M

Error on the side of having slightly too much

Yep. I've had two boats.. at least ... that I wish had more power. No problem in flat water. One was a 31 footer with a 13 hp volvo. All the "old school" calculations said that power was enough for the boat. Engine would over heat if pushed hard, even though propped properly. The other was the HC38 seen in my Avatar, 30 HP (not!) Yanmar. In a heavy wind chop that boat wouldnt move out of it's own way under power!

[O.C.Diver]

One of the things to keep in mind is that a given HP may be generated over a wide range of RPMs and the fuel consumption may vary significantly for that same HP. Most published numbers for HP at a given RPM represent the maximum , not necessarily the most efficient. As RPM increases the engine sucks in more air changing the fuel to air ratio / mixture. Generally, higher RPMs (more air intake) produce more efficiency or HP for the same fuel consumption. Also, higher RPMs increase the coolant circulation through the motor and GPM through the raw water system. Obviously there is a limit to optimal long term RPM and a diminishing return near maximum continuous for your motor.

In my own situation, flattening my wheel 1" increased the RPM by 200 for the same speed and reduced the fuel consumption by 14% to 16% depending on load! Trying to decide whether or not to take out another 1".

Ted

[Tony B]

Originally Posted by europaflyer

................The only reply really against the trend was Tony B, I respect your opinion but think it might be a little skewed seeing a powerboat as your picture?
We all get our info from different sources. I have always seen the HP required as 2-3 HP per Long Ton. I metioned in my last post that the HP req'd was 2-3 HP per Long Ton. I always err on the safe side and used 3. Anyway, my opinion is not skewed or influenced by my Avitar. Last week my Avitar was a 39' Allied Ketch which is now for sale. I am in transition of going to the dark side. Something that just comes with age.
Also, the waterline is only 24ft which makes a difference.
I didn't know your boat so I guessed at it for example purposes only. With a 24' waterline, your theoretical hull speed is 6.566 kts. Missed it by .334 kts. How could I have ever made such a large mistake? ...........

I think my answer is actually fairly accurate. I came up with 15.4 HP required to push your boat, fuel, water, passengers etc at almost hull speed. The 15.4 HP ia actually calculated at what is needed with nothing to spare. If you only had a motor that only produced 15.4 HP, it would have to run at full throttle with absolutely nothing to spare. So, the manufacturer chose to give you a 20HP motor. That is approx 30% reserve power.

From your original post, I got the impression that you now have an 18HP motor and wanted to repower. And now you are thinking that going from 18Hp to 20HP is the way to go because that is what the manufactirer gives you.
Oh well, sorry I misunderstood the question.

[Tony B]

Originally Posted by Cheechako

... All the "old school" calculations said that power was enough for the boat. Engine would over heat if pushed hard, even though propped properly. .......!

In the year 2000, I bought a 1984 Catalina 30. The average price for that boat was around $25-$30K. It came with a (I think) a 25HP diesel. That motor was fine for the normal conditions in the Gulf of Mex. Some of the Catalina 30's in the early 1980's had options for I think 14 and 18HP engines. Those sold for 1/2 the price or less even though in good condition - yup, anyone who knew anything knew that you could use the lower HP engines for in and out of slip only and only on a nice day.
The formula I use is for what is actually required to move the boat and is certainly not what is recommended even though some manufacturers used the lower HP engines to help keep the cost down and the sales high.They can get away with this easier with sailboats than power boats because "hell, you will be sailing and other than in and out fo the slip you wont even need the motor". Little do they know.

[Cheechako]

"I have always seen the HP required as 2-3 HP per Long Ton..."
I would take this as the HP needed for an auxilary to get you out of the marina. This is the mistake I made on my 31 footer: Displ=10500, long ton is... 2240? = 14HP required using the 3hp/long ton. This is not enough for Normal cruising etc. Persoanlly I would nearly double it. 30 hp at max rpm for a 11000 lb boat is not unreasonable at all.

[donradcliffe]

Back to the original poster, the fuel consumption curves are for liters per hour versus RPM with a prop which is matched to require full rated engine power at the rated RPM. Do not make the mistake of comparing the fuel consumption at less than rated RPM with the MAXIMUM horsepower available versus RPM.

It is based on different assumptions--- the horsepower vs RPM which the fuel consumption curve is based on drops off much faster at lower RPM.

As the other posters have said, there are a lot of tradeoffs to consider on engine size--reserve, wear, price, weight--but with the right prop most engines will get about the same gas mileage.

[barnakiel]

@ VP spare parts availability and prices:

Having sailed some, we have never found spares availability an issue. There was always some sort of Volvo dealer nearby or else a shop willing to order what we want. So that's for Volvo, I cannot speak for other makes.

Prices? Well, in 10 years of use (on an initially 20 y.o. engine) we never needed spares to the point that we got stuck. We replaced only 2 major parts in this time: the fuel pump and the escape elbow.

I am not quite sure new VP engines break down so often that we need a VP dealer in every cruising spot. The ones I use on other people's boats seem to fare pretty well.

So, in the sailing boat aspect, passing a VP on the basis of any of these two a/m factors could be a mistake.

b.

[Tony B]

Originally Posted by Cheechako

"I have always seen the HP required as 2-3 HP per Long Ton..."
I would take this as the HP needed for an auxilary to get you out of the marina. This is the mistake I made on my 31 footer: Displ=10500, long ton is... 2240? = 14HP required using the 3hp/long ton. This is not enough for Normal cruising etc. Persoanlly I would nearly double it. 30 hp at max rpm for a 11000 lb boat is not unreasonable at all.

Yes and yes. That is what my feelings were in my first post. With a small engine like that, double the calculated HP.
My Catalina 30 was around 11,000 lbs and came with a 25HP. It was pretty good for 85% of the time. There was that 5% of the time when 30HP would have been better. Usually when motorsailing against the wind and out of beer.

[Tony B]

All things being equal, and a full displacement hull, fuel consumption is more a function of Theoretical Hull Speed than anything else.
With a semi-displacement hull the first major factor in fuel consumption is still theoretical hull speed. Once you pass theoretical hull speed, the next major factor is you planing speed.

[Cheechako]

Re: Volvo. Yeah, reportedly the parts are expensive, but they seem like very well built, heavy duty engines. Maybe you dont put much parts on them! The old MD series were hard to beat with a stick.... the newer ones, at least in the higher HP's were significantly heavier than the Yanmar equivalent. We installed a few where I used to work. Solid engine...

[Tony B]

In your original post, Post #1, you state "The boat needs around 10hp by my estimation to be pushed at a nice cruising speed" and it appears all of your references on Post #1 are based on 10HP.
Post #5, says your boat is 32' and 9,500 lbs.
Post #19 you state 2HP per 1,000 lbs and round up to 20HP. That makes mathematical sense but disagrees with your Post #1.
From your remarks, I gather you have a sailboat. What make and model do you have?