Engine Size

[Alan Wheeler]

Yes it fits right in the middle, so don't panic. On the Light end of the power to displacment rule of thumb, you should have 56HP and heavey end you could go for 70HP. So 62 is an OK choice.

[Capn Jimbo]

Yes, yes I know this is an old thread, but the question is one still asked, and still requires a qualified answer - which rules me out, lol.

However, it does NOT rule out Hal Roth in his wonderful book "How to Sail Around the World...". In it he states:

"I believe 2 hp per ton displacement is adequate power... When Tom Colvin built his 42-foot 'Gazelle' (11 tn), he installed a 10-hp Saab that gave the boat a steady 4 to 5 knots in calm water.

I have seen the same design with a 50 hp Perkins 4-108. Of course, the larger engine drives the yacht faster and can stop the boat more quickly at a dock. However, the 4 cylinder Perkins is far heavier and larger, uses at least three times the fuel, and is more costly in every regard.

Is it really necessary to go 7 knots instead of 5? I've heard owners complain that they needed a larger engine because they 'had to motor into a chop'. Now if there's a chop, it suggests wind.

Why not put up some sails?

Twelve and fifteen hp engines were ample for our first 'Whisper's' 6 tons. On her circumnavigation, we had a 1 cylinder, 12 hp Farymann that was perfect. It had reasonable power for maneuvering and was easy on the fuel. It had an electric starter, but the engine was a cinch to hand crank if the battery was down.

(Better yet) raw water cooling seems quite adequate for these small engines and is cheaper and simpler."

The aforesaid Tom Colvin felt strongly that 1/2 hp per ton of displacement was fine for an ocean going yacht, but added that 1 hp per ton was more appropriate for a coastal cruiser who face tide, nasty currents and who may be in a hurry.

Francis Kinney in "Skene's Elements of Yacht Design" recommends about 2.5 - 3.5 hp per ton. And ccording to Don Casey "...most boatbuilders today recommend about 4 - 5 hp per long ton.

So...

Colvin: 1/2 to 1 hp/ton
Roth: 2 hp/ton
Kinney: 2.5 to 3.5 hp/ton
Builders: 4 to 5 hp/long ton

And let's not forget Pardey: a 17' skulling oar...

[Lepke]

Smaller engines usually run at higher rpms and often have turbos to cram even more air in the cylinders. Higher rpm means more wear. Turbo higher temps means more wear. Many small engines do not have sleeves so rebuilds require more machining and can make them too expensive to overhaul.
An underpowered boat is a danger in high winds, currents, big waves, etc. Even lakes have tides if they are big enough.
If you are thinking of replacing gas engines with undersized diesels, the fuel savings would take years to come out ahead. You'd be better off to slow down the engines you have.

[barnakiel]

I think a small engine is a great idea and is 99% as good a choice as a big engine.

That 1%, however, seems to be exactly what you mentioned - e.g. adventures with a shoreline quickly approaching you from the lee. Then you pray for plenty of hp.

Otherwise I am into small engines.

BTW re noise - I noticed heavier and well insulated engines are quieter than lighter and poorly insulated ones ... ;-) "Size does not matter".

b.

[barnakiel]

Originally Posted by Boracay

Is it really necessary to drag what is an extra 500Kg halfway

(...)

There is very little weight penalty with extra hp purchased. To say engines much stronger are not much heavier. Bummer.

IMHO a boat that sails well and that is sailed by a keen and observant sailor can do without a big engine or even without an engine. Look at Webb Chiles' present adventure.

When I started sailing, my early boats did not have engines. Then again, we had docks, not marinas, and we never anchored for longer than a lunch ...

I would be comfortable with a small ('too smal') diesel. I think every sailor is different and we all have our own takes on what makes us feel safe in our boats.

b.

[valhalla360]

I think it comes down to WHY are you going smaller.

Financially, it makes little sense. Fuel consumed is mostly a function of power produced and the big engine will be putting out low power most of the time. The big engine may cost more up front but not being stressed to the max constantly should last longer. Fuel tanks will be about the same size. Also, some of the extra cost of the bigger engine will probably be saved if you can match the existing engine mounts where as a much smaller engine may require substantial work to prep for installation.

I think the varying numbers reflects when they came out. In the 1930's a 50hp diesel was massive and expensive by current standards, so they justified a 25hp by lowering the requirement. As cheap and reliable higher HP engines have become more available, they have been used. Can you get by with less...sure but in the modern world, there is no reason.

[Capn Jimbo]

Originally Posted by valhalla360

I think it comes down to WHY are you going smaller.

Financially, it makes little sense. Fuel consumed is mostly a function of power produced and the big engine will be putting out low power most of the time. The big engine may cost more up front but not being stressed to the max constantly should last longer. Fuel tanks will be about the same size. Also, some of the extra cost of the bigger engine will probably be saved if you can match the existing engine mounts where as a much smaller engine may require substantial work to prep for installation.

I think the varying numbers reflects when they came out. In the 1930's a 50hp diesel was massive and expensive by current standards, so they justified a 25hp by lowering the requirement. As cheap and reliable higher HP engines have become more available, they have been used. Can you get by with less...sure but in the modern world, there is no reason.

The power requirements in my earlier post were cited and taken from Hal Roth, published in 2003, a modern recommendation for modern times.

Secondly the notion that "The big engine may cost more up front but not being stressed to the max constantly should last longer." is exactly backwards. The consensus would be that for longest life and best fuel consumption a sailboat diesel should be run at 80% of power and 90% of max prop speed. Most surveyors actually run this test.

The caveat: the internet is the best source for both the best and sadly for the most harmful advice. Readers beware, and be skeptical of all, including this post.

[valhalla360]

Originally Posted by Capn Jimbo

The power requirements in my earlier post were cited and taken from Hal Roth, published in 2003, a modern recommendation for modern times.

Secondly the notion that "The big engine may cost more up front but not being stressed to the max constantly should last longer." is exactly backwards. The consensus would be that for longest life and best fuel consumption a sailboat diesel should be run at 80% of power and 90% of max prop speed. Most surveyors actually run this test.

The caveat: the internet is the best source for both the best and sadly for the most harmful advice. Readers beware, and be skeptical of all, including this post.

I agree with the last paragraph, particularly as it applies to your most recent comment.

If we were talking about running a 50hp engine at 3hp (such as for charging batteries day after day, month after month never running it hard enough to get up to temp) I would agree. That's hard on an engine but not really much better than a 30hp engine at 3hp.

But if you are regularly having the 50hp engine put out 28hp, assuming all else is equal, that engine is going to last longer than a 30hp engine putting out 28hp because it has enough load to get up to temp which makes the engine happy but less stress.

As far as fuel efficiency, yes, the smaller engine will do better but by such small amount as to be irrelevant. Your quote of 3 times the fuel consumption, shows you haven't got a clue. For the same HP generated, you will struggle to show a statistically different fuel consumption rate.

In the end, retrofitting a smaller engine is likely going to cost as much as getting a matching larger engine.

I'm still trying to understand what you hope to gain but if you really want an underpowered boat, get an underpowered boat.

[zeehag]

boracay.
i have a 54 hp perkins(approx) in my formosa 41, allegedly 28000 pound displacement, and i covet a 4-236. if you have opportunity to have larger engine, do it. uphill aint fun with underpowered anything. especially with weather. and some weather ye really want to be able to exit before it arrives. best of luck to ye.
oh yeah and sails blow out, even new ones, so...... i managed to make 2.5 kts uphill with blown out genoa, which seems to be my driving sail. main didnt drive much at all. engine did almost all the work, as mizzenmast was fail from cane patricia.


oh, yes i covet 4-236.....

[Terra Nova]

TN is 12-tons displacement with 5-tons of that being internal lead ballast. She has a low-rpm Yanmar 3QM30, rated at 30 hp continuous, and a fixed, 2-blade prop. So far I have not wished to have a bigger engine; this one barely fits within the island, galley sink cabinet.

[Capn Jimbo]

Originally Posted by valhalla360

I'm still trying to understand what you hope to gain but if you really want an underpowered boat, get an underpowered boat.

When this poster quotes data from the 1930's, I could pose the same alleged concern, namely what he/she hopes to gain. The sources I quoted are all competent and well founded, from reliable and trustworthy sources. It's clear that there is a tendency for the alleged and mythical "modern" sailor to overpower, and for the builders to pander to them. I also stand on relating an apparent consensus that both the best life and economy are at 80% of HP and 90% of matched prop speed. The term "modern" is unclear, though I suspect Hal Roth's book, circa 2010 is modern enough for me.

Personal opinions are welcome, but repeating them does not make them any more convincing. Surely all posters have the right to their widely varying opinions, however well founded or not, personal or not - as they may be. I will end this post the very same way:

"...the internet is the best source for both the best and sadly for the most harmful advice. Readers beware, and be skeptical of all, including this post."

My advice: less coffee. Relax. Breathe. And as always, keep calm and carry on, mate...

[valhalla360]

Originally Posted by Capn Jimbo

I also stand on relating an apparent consensus...
...

Since you have made the pronouncement that there is "consensus" multiple times, clearly you have stated it enough times that it must be true.

Go get that underpowered engine and enjoy your lower resale value, shorter engine life, same fuel economy all while paying about the same amount for the engine replacement...kind of makes you wonder why you bothered to ask the question since there is already clear consensus.

[Capn Jimbo]

Parsing, misrepresentation and personal bloviating/repetition are never valid argument. If posters cannot accept that there are a variety of opinions that differ from their own, that is a sad day.

The my way or the highway approach tends to be offputting and counterproductive. Far better to find and quote those authors and resources that are - by consensus, lol - widely accepted, or to express one's personal opinion and leave it a that.

I am far from not alone here in disagreeing with the "bigger is better" approach, when there are any number of authorities and other qualified sources who establish that adequate HP is somewhat less than some would lead us to believe (not least the pandering builders).

I'll say it once again:

"...the internet is the best source for both the best and sadly for the most harmful advice. Readers beware, and be skeptical of all, including this post."

My advice: less coffee. Relax. Breathe. And as always, keep calm and carry on, mates...



*********
But in the interest of countering misrepresentation directed at (and quoting) moi:

"If we were talking about running a 50hp engine at 3hp (such as for charging batteries day after day, month after month never running it hard enough to get up to temp) I would agree. That's hard on an engine but not really much better than a 30hp engine at 3hp."

My point exactly, apparently this poster agrees, and is thus making a non-point.

"But if you are regularly having the 50hp engine put out 28hp, assuming all else is equal, that engine is going to last longer than a 30hp engine putting out 28hp because it has enough load to get up to temp which makes the engine happy but less stress."

Misrepresention. The actual consensus by research cited by me was in effect "...the best fuel economy and engine longevity is achieved at or around 80% of HP and about 90% of matched prop speed". This poster's creative and frivolous example tries to compare a large engine at 56% HP vs a smaller engine at 93%?! This is a straw man, completely ignoring the claim it presupposes to argue against, ergo fails on its face.

"As far as fuel efficiency, yes, the smaller engine will do better but by such small amount as to be irrelevant. Your quote of 3 times the fuel consumption, shows you haven't got a clue. For the same HP generated, you will struggle to show a statistically different fuel consumption rate".

This "argument"too is created from whole cloth and is entirely irrelevant. The claim of "3 times the fuel consumption" was never made by me. Made up argument, not to mention the poster fails to document his/her "such a small amount". Not satisfying, not convincing. Citations and quotes from respected sources might help but were absent.

"In the end, retrofitting a smaller engine is likely going to cost as much as getting a matching larger engine."

Yet another made up argument against a claim never made by me. My post was entirely and exclusively to put forth a range of qualified and cited recommendations made by respected authors and other qualified resource for what might constitute enough power. Never, ever did I suggest or imply that we should all run out and replace our engines. If this poster has a bone to pick, it's with Hal Roth, Colvin, Kinney and the builders.

My sincere apologies for quoting respected authorities, lol.

Friends, this is supposed to be a respectful and open-minded website and for the most part it is. We all have the right to express our opinions, or to cite opinions of others without having defer to the my way or the highway types who not only express an opinion (good) but then repeat it ad nauseum in opposition to the many here who may have disagreed. All viewpoints and opinions are welcome, but brow and breast beating to dominate and sell one's personal opinion as "the best", is not.

Such activity does not become mutual and respectful dialogue...

[a64pilot]

Two motors, both rated to run a max continuous RPM of 3600, both engines propped to just barely reach 3600 at full throttle.
One run 99% of the time underway between 2,000 and 2,200 RPM, (58%) the other run 99% between 2,800 and 3,000 (80%)

The one run at the lower RPM will outlast the one run harder. That is my experience, engines run between 50% to 75% seem to do the best, Ones run much lower do well also assuming they are run up at higher power ever so often.
Example a Sport Fisherman may troll at not much above idle all day and night, but almost always will run them up on the way out and on the way home.

But it's load more than RPM, but we are assuming properly propped and if so load and RPM are pretty consistent.

[barnakiel]

blah blah blah consensus or not

Auxiliary is just a sailor's choice like anything else.

Boats that sail well and that are sailed mostly in open not too tidal waters clearly do not require 5hp per ton.

Boats that are more into the motor-sailor territory or ones that sail in restricted rocky water with plenty of tide may ask for that bigger end of the scale diesel. Not a must, but a wise compromise if one can afford the ticket.

etc.

Needles to say the better the sailor, the less need for engine hp. And who am I to tell anyone what sailor they are.

Cheers,
b.