Engine sizes.

[Allan]

Another thread has got me thinking. Is there a basic formula for working out the size of engine required by different yachts? I have 50hp in my 33ft, I've sailed a 50ft with 18hp and I know many boats with various sizes of engine. Recently I lent my 4hp outboard to a friend of mine who put it onto his 30ft motor cruiser and said it push him along at 3kts.
Allan

 

[sailorman]

Another thread has got me thinking. Is there a basic formula for working out the size of engine required by different yachts? I have 50hp in my 33ft, I've sailed a 50ft with 18hp and I know many boats with various sizes of engine. Recently I lent my 4hp outboard to a friend of mine who put it onto his 30ft motor cruiser and said it push him along at 3kts.
Allan

4 hp per ton displacement

 

[Tranona]

Various "rules of thumb" - Sailorman's is one, but 30 years ago 3bhp per ton was more common - simply because engines had less power and lower motoring speeds were acdceptable - indeed for many just being able to motor was a bonus.

There are several more sophisticated formulae, probably best known are the ones by Nigel Calder in his book (forget the title) and Dave Gerr.

The objective of the formula is to determine hp required to give maximum displacement speed at maximum engine power making allowance for typical transmission losses and normal prop efficiency.

Both of those formulae will show your boat is grossly overpowered and is likely to give a recommendation of between 30-35 rated hp - that is just a bit more than the designer originally specified - but he probably would have gone up a bit from the 28hp normally used if there had been a suitable engine available at the time.

There has been a tendency in recent years to overpower boats, particularly re-engine jobs because the incremental cost in terms of both money and space of the next size up is usually small. This leads to a view that "more is better" because it doesn't "cost", but the extra power cannot be used.

 

[Allan]

Many thanks, yes I know mine is overpowered. most of the standard models had 42hp Mercedes engines. I just wondered if there was an "official" formula.
Allan

 

[Twister_Ken]

4 hp per ton displacement

Works for me. 5 ton boat, 20 Yanmar horses

 

[Tranona]

Many thanks, yes I know mine is overpowered. most of the standard models had 42hp Mercedes engines. I just wondered if there was an "official" formula.
Allan

Yes, as suggested there is a "standard" model - the two I mentioned are widely used. There is also a consensus amongst mainstream boat builders of appropriate sizes for particular sizes and types of boats as you will see if you look at specifications. At a particular LWL (which determines speed) you find quite widely varying displacements which can account for differences in power requirements. You also find that boats on the margin tend to have options which can be quite different. So my Bavaria 37 came as standard with a Volvo 30hp which is perfectly adequate, certainly for the Med where it was mostly used. It also had an option of 40 (the next size up) which might be more suitable if the boat was to be used fully laden in heavier conditions. It is way overpowered in normal use and a bit of a beast at low speeds. On the other hand an HR 37 has nearly 50% greater displacement than my boat, so not surprisingly has a 50hp engine as standard. Probably, again a bit too much, but the alternative is 40hp, not quite enough.

Think your boat got its 42hp because there was nothing suitable at the time from Volvo in the 30-40hp range. In the same way the Centaur got a 23hp in most cases because that was the engine available from Volvo. Most re-engines have gone for smaller engines.

One of the features of the latest Japanese industrial engines is the much finer grading of hp making it easier on the one hand to optimise the power for a particular application - and provide an opportunity to increase power with less penalty than in the past when engine ranges tended to jump in 10hp increments. In a small sailing boat that leads to doubling the power when probably just a 30-40% increase would be all that is required.

 

[Allan]

Many thanks for that, I will wait until I'm a little more sober before I try to digest it fully. It occurs to me that, if you don't require hull speed, you could reduce the power requirement quite substantially.
Allan

 

[Twister_Ken]

Many thanks for that, I will wait until I'm a little more sober before I try to digest it fully. It occurs to me that, if you don't require hull speed, you could reduce the power requirement quite substantially.
Allan

You could, but if you were trying to punch into wind and waves you might find your ground speed was impressively near 0kts.

 

[Allan]

Many times, for me, that would not be too much of a problem. It may also just mean that you had made a mistake whilst doing the tide calculations.
Allan

 

[ukmctc]

Another thread has got me thinking. Is there a basic formula for working out the size of engine required by different yachts? I have 50hp in my 33ft, I've sailed a 50ft with 18hp and I know many boats with various sizes of engine. Recently I lent my 4hp outboard to a friend of mine who put it onto his 30ft motor cruiser and said it push him along at 3kts.
Allan

its all power to weight and hull speed, the boat can go slower but never faster than the hull speed no matter what engine it has. See manufacturer (boat) for engine size recommended.

 

[reeac]

Many times, for me, that would not be too much of a problem. It may also just mean that you had made a mistake whilst doing the tide calculations.
Allan

Not necessarily. If you want to exit our estuary [the Ore] on a rising tide and want a couple of hours of fair tide behind you when heading South then you have to be able to make progress against ca. 5 knots of tide for what feels like half a mile but is probably less. Our Sadler 29 with VP2002 [18 bhp] can do it but needs full power to do so.

 

[Allan]

Not necessarily. If you want to exit our estuary [the Ore] on a rising tide and want a couple of hours of fair tide behind you when heading South then you have to be able to make progress against ca. 5 knots of tide for what feels like half a mile but is probably less. Our Sadler 29 with VP2002 [18 bhp] can do it but needs full power to do so.

Yes, I'm sorry for that, I should not post when I come back from the pub. I sail mostly in the Bristol channel where the tides are really good but you don't try to sail against them. Are you able to leave the Ore at low tide?
Allan

 

[stevensuf]

probably about 3hp per ton will get you to hull speed in fairly benign conditions, make that 5 hp per ton, with wind and waves on the nose and one bumpy ride!

 

[reeac]

Yes, I'm sorry for that, I should not post when I come back from the pub. I sail mostly in the Bristol channel where the tides are really good but you don't try to sail against them. Are you able to leave the Ore at low tide?
Allan

Not in a cruiser. The shallowest point on last year's channel [it changes every year] was at about chart datum.

 

[benlui]

i installed 21hp (again) last year, my boat comes in about 4 tonnes loaded up. I can reach hull speed no problems and very efficiently too without working the engine hard. I almost changed my mind and went with the 18hp option but now, i'm glad as i feel 21hp was just perfect for my type of boat, and weight.

 

[Ex-SolentBoy]

Various "rules of thumb" - Sailorman's is one, but 30 years ago 3bhp per ton was more common - simply because engines had less power and lower motoring speeds were acdceptable - indeed for many just being able to motor was a bonus.

There are several more sophisticated formulae, probably best known are the ones by Nigel Calder in his book (forget the title) and Dave Gerr.

The objective of the formula is to determine hp required to give maximum displacement speed at maximum engine power making allowance for typical transmission losses and normal prop efficiency.

Both of those formulae will show your boat is grossly overpowered and is likely to give a recommendation of between 30-35 rated hp - that is just a bit more than the designer originally specified - but he probably would have gone up a bit from the 28hp normally used if there had been a suitable engine available at the time.

There has been a tendency in recent years to overpower boats, particularly re-engine jobs because the incremental cost in terms of both money and space of the next size up is usually small. This leads to a view that "more is better" because it doesn't "cost", but the extra power cannot be used.

I think Tranona is spot on.

When we were calculating engine size for the first Rustler 44 we built an Excel model from the Nigel Calder/Dave Gerr data. We also asked the engine manufacturer to recommend and engine size. We did the same thing to the prop supplier.

Interestingly, all of the above and the boat designer were in agreement as to the required hp. The same engine could have given 15hp more with an intercooler option, but the extra horsepower would have needed a bigger prop and would have ent the engine running inefficiently to achieve hull speed.

The relationship between engine, prop and gearbox is important to get the ability to reach hull speed at max rpm. Doing this gives befits in fuel consumption, noise reduction and engine wear.

I recommend you buy the book. Do a bit of simple arithmetic and feel comfortable with your own calculations.