Engine Size

[StuM]

Originally Posted by Capn Jimbo

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.

You did not directly made the claim. However, you quoted it in Post #17 in support of your argument. Are you now disavowing what you quoted?

[Capn Jimbo]

Originally Posted by StuM

You did not directly made the claim. However, you quoted it in Post #17 in support of your argument. Are you now disavowing what you quoted?

No, no and no. Absolutely not, and why should I? Having a bad day?

Roth's experience and opinions are his and his alone. Obviously. To the contrary, I expressed no personal opinions, but felt it useful to name, cite and quote a range of differing opinions. First I quoted Hal Roth, who expressed his vast experience, from his famous 2010 book "How to Sail Around the World. I then quoted Colvin, then designer Francis Kinney, and finally some research expressing what appeared to be a consensus of boat builders.

None of these agreed.

These were offered up - not to express my own opinion at all - but to provide a range differing opinions, so for this poster to engage in a personal attack on me, creatively parsing out Roth's quoted opinion is somehow mine, is bizarre and and in my opinion, a cheap, forced and completely unwarranted criticism.

But have at it Hoss, if it helps you sleep better.

For those who wish to view the entire post again - so as to judge the worth of Mr. Stu's rather novel criticism, here you are:

**********

Post #17 (in toto):

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...


*********

In sum a clear, informative and unbiased post that expressed no personal opinion whatsoever.

In closing, I suggest those who can't stand the expression of cited expert opinions that may differ from their own, might try to resist posting cheap shots at the party who happens to merely post them. I made it clear from the get go, that I had no personal expertise or opinions to offer. I happen to respect the sources quoted, feel that those expert opinions - however they all do differ - have great value to the forum.

It appears though, that no good deed goes unpunished. Lighten up my friend. Try decaf, or better yet a Pusser's Blue Label. Relax. Breathe.

[Jim Cate]

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

If the "experts" quoted above can differ by a factor of ten (1/2 up to 5 hp/ton), why is it unreasonable for us to also disagree with either your or their opinions?

This is not an attack on you; this is an attack on the logic presented.

Jim

[kmacdonald]

1 HP per 750 pounds. Nothing else will work as well.

[markpierce]

Sailboats have small propellers. Lots of horsepower won't help. Or do you have a motorsailer?

[Capn Jimbo]

Originally Posted by Jim Cate

If the "experts" quoted above can differ by a factor of ten (1/2 up to 5 hp/ton), why is it unreasonable for us to also disagree with either your or their opinions?

This is not an attack on you; this is an attack on the logic presented.

Jim

Au contraire.

Although it's perfectly fine - of course - to disagree with the opinions of these experts, there is no cause whatever to disagree with "my opinion" for one simple reason: I didn't express one. In fact, that post was opened with this statement "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.". I then simply shared a variety of differing expert opinions.

Point made (again) and enough said. I expressed no opinions to counter. Period. So let's keep it real; I do agree with you Jim - it is perfectly valid then to examine Roth's position:

*********


At issue: is Roth's claim that a 50 hp Perkins 4-108 uses "at least 3 times as much fuel" realistic?

Roth:

“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?”

Is Roth a fool? Lying? Purposely misleading the reader for some conspiratorial purpose? Using an isolated fact for misleading purposes? Unrealistic? Inexperienced? I think not.

So let's take a rough cut and examine Roth's very real world experience.

According to John Vigor in his book “Things I'd wished I'd known...”, Vigor states “...a diesel engine needs roughly 1 gallon per hour for every 18 HP you're using.” In accord with this, and for a best case example, let's push the 11 ton design cited by Roth, first with the 10hp Saab running at say at a recommended maximum 80% or 8hp for 5 knots. Based on Vigor this would use about 4/10ths of a gallon per hour.

For the sake of argument, let's say the 50 HP Perkins was run at this thread's suggested minimum of 50% of continuous power or 25hp, for 7 knots. Thus the Perkins would use no less than 1.4 gallons per hour or 3-1/2 times as much fuel. If we take as gospel one of our local critics suggestions – a minimum 58% of power - the 50hp Perkins would use even more.

Whether we agree or disagree with this particular analysis, it's close enough to Roth's reported experience, that his claim of 3 times as much fuel consumption seems quite fair enough.

Remember too that Roth is also critical of the need to go 7 knots (with a big engine) when he demonstrated that the same design with an engine 1/5th of that size will push it along just fine at 5 knots. As for the issue of “motoring into a chop” the He correctly points out that a chop indicates “wind”, ergo “Why not put up some sail?”

By the way, please take note that none of the above represents my personal opinions, which remain completely unqualified, thus and and as before I won't share them, lol. I know better...

Your witness...

[Jason Flare]

I read somewhere that a Perkins 4-108 would/should burn about .85 gallons/hour.

Mine seems to burn slightly less than that and I run it over 50% power.

1.4 gallons/hour at 50% seems very high.

[gjordan]

I suspect that the reason builders recommend large engines , is "THAT IS WHAT SELLS". Marketing dictates so much of the boat building trade because, as this thread points out, Most people seem to think that you need large amounts of horsepower to go cruising. This bigger is better idea is permeated into our society and you just have to look at new cars with 5 times as much HP as will ever be needed, or motorcycles, or anything else where someone feels better with more power. Is it really needed??? That is a matter of opinion (just like this whole thread) but people that believe it, wont feel safe without that extra HP, and marketing people and boat builders know this. Of course, my post is just another opinion. _____Grant.

[valhalla360]

Originally Posted by Capn Jimbo

No, no and no. Absolutely not, and why should I? Having a bad day?

Roth's experience and opinions are his and his alone. Obviously. To the contrary, I expressed no personal opinions, but felt it useful to name, cite and quote a range of differing opinions. First I quoted Hal Roth, who expressed his vast experience, from his famous 2010 book "How to Sail Around the World. I then quoted Colvin, then designer Francis Kinney, and finally some research expressing what appeared to be a consensus of boat builders.


In sum a clear, informative and unbiased post that expressed no personal opinion whatsoever.

In closing, I suggest those who can't stand the expression of cited expert opinions that may differ from their own, might try to resist posting cheap shots at the party who happens to merely post them. I made it clear from the get go, that I had no personal expertise or opinions to offer. I happen to respect the sources quoted, feel that those expert opinions - however they all do differ - have great value to the forum.

It appears though, that no good deed goes unpunished. Lighten up my friend. Try decaf, or better yet a Pusser's Blue Label. Relax. Breathe.

(Emphasis added)

When you claim you source as having vast experience, famous in the subject area and don't disagree with the statements, you imply you agree with the source. Hardly unbiased when claiming "vast experience". (Never heard of Roth or his world famous book by the way.)

Of course, then you follow up by trying to defend his incorrect statement with an example that someone with the engine in question thru personal experience has shown to be wrong. But more importantly, you change the assumptions to suit your needs. You compare an engine putting out 8hp to an engine putting out 25hp...gosh you really think it might use more fuel..and not be a comparable example, since you can always cut the fuel consumption on the larger engine by simply throttling back.

I do agree cheap shots are not appropriate. Hmmmm:
"...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."

Might want to save a little of that decaf you are proscribing everyone who disagrees with you for yourself.

[kmacdonald]

1 HP per 1000 pounds if you cruise at 80% of hull speed or less.

[kmacdonald]

1 HP per 500 pounds for a motor sailor like an Island Packet, Westsail, or other slow and heavy tub.

[TeddyDiver]

Originally Posted by Capn Jimbo

Is Roth a fool? Lying? Purposely misleading the reader for some conspiratorial purpose? Using an isolated fact for misleading purposes? Unrealistic? Inexperienced? I think not.

So let's take a rough cut and examine Roth's very real world experience.

According to John Vigor in his book “Things I'd wished I'd known...”, Vigor states “...a diesel engine needs roughly 1 gallon per hour for every 18 HP you're using.” In accord with this, and for a best case example, let's push the 11 ton design cited by Roth, first with the 10hp Saab running at say at a recommended maximum 80% or 8hp for 5 knots. Based on Vigor this would use about 4/10ths of a gallon per hour.

For the sake of argument, let's say the 50 HP Perkins was run at this thread's suggested minimum of 50% of continuous power or 25hp, for 7 knots. Thus the Perkins would use no less than 1.4 gallons per hour or 3-1/2 times as much fuel. If we take as gospel one of our local critics suggestions – a minimum 58% of power - the 50hp Perkins would use even more.

Whether we agree or disagree with this particular analysis, it's close enough to Roth's reported experience, that his claim of 3 times as much fuel consumption seems quite fair enough.

Remember too that Roth is also critical of the need to go 7 knots (with a big engine) when he demonstrated that the same design with an engine 1/5th of that size will push it along just fine at 5 knots. As for the issue of “motoring into a chop” the He correctly points out that a chop indicates “wind”, ergo “Why not put up some sail?”

He is not lying so is a fool indeed.. The problem with this reasoning is the other motor is (small engine) is driven 5kn with propeller suited for the engine and boat and the other one driven faster and underpropped. Not comparable situation at all.

Oversizing the engine must be considered carefully. Following the general advice of propeller recommendations (full throttle, max rev etc) is road to he** with oversized engine. There are two ways to get along in this. The other one is to overprop so you get about 70% loading for the engine at cruise speed, the speed will be around 1kn below hull speed rev's at half of the max and low fuel comsumption. The caveat is that you cannot get the top rev's from the engine anymore. Better solution is to get a propeller with adjustable pitch as Bruntons autoprop or even better a true CPP, controllable pitch propeller, and allways drive with best effiency at all speeds. There's a lot more into this but it needs a book...

BR Teddy

[valhalla360]

Originally Posted by gjordan

I suspect that the reason builders recommend large engines , is "THAT IS WHAT SELLS". Marketing dictates so much of the boat building trade because, as this thread points out, Most people seem to think that you need large amounts of horsepower to go cruising. This bigger is better idea is permeated into our society and you just have to look at new cars with 5 times as much HP as will ever be needed, or motorcycles, or anything else where someone feels better with more power. Is it really needed??? That is a matter of opinion (just like this whole thread) but people that believe it, wont feel safe without that extra HP, and marketing people and boat builders know this. Of course, my post is just another opinion. _____Grant.

Agreed, if we are talking about shaving 20-30% off the engine size, you can still get reasonable performance but the cost savings would be negligible, particularly if you retrofit a smaller engine that doesn't match the existing engine mounts.

When the quoted expert with "vast experience" is claiming 10hp is fine for a heavy 42' boat and then supports it saying in the best possible conditions, with the throttle firewalled, it can do 4-5kts, it's a whole different ballgame. Unless you are going to compare it to something silly like trying to use a sculling oar on a 42' boat.

[StuM]

“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?”

> Is Roth a fool? Lying? Purposely misleading the reader for some conspiratorial purpose? Using an isolated fact for misleading purposes? Unrealistic? Inexperienced? I think not.

I think so

Specifically, using an isolated fact for misleading purposes and unrealistic.

> For the sake of argument, let's say the 50 HP Perkins was run at this thread's suggested minimum of 50% of continuous power or 25hp, for 7 knots.

I haven't seen anywhere in the thread a suggested minimum of 50% of continuous power. Are you perhaps confusing power with RPM.

Let's look at the real figures for a Perkins 43-108 with a typical sailboat auxiliary prop that just achieves the rated max Intermittent Speed:

All figures from the Power curves in the 4-108 Manual and applying your 1gph /18HP figure

(Max Speed for High Speed Pleasure Craft 4000RPM 50HP N/A)
Max Intermittent Speed - 3600 RPM = 47HP = 2.6GPH.
Max Continuous Speed - 3000 RPM = 28 HP = 1.55 GPH
80% of Max Continuous Speed (the "sweet spot") = 2400 RPM = 15 HP = 0.83 GPH
50% of Max Continuous Speed - 1500RPM = 4HP = 0.22GPH

That "sweet spot" would be about that 7 knots. So 7 knots for 0.83 GPH.

The SAAB? Hammering away at 80% rated power which would close to 100% continuous rated RPM and more like 0.5 GPH for 4 -5 knots?

So;
10 HP SAAB about 1.11 gallons per nm.
Perkins 4 -108 about 1.18 gallons per nm.
Not exactly "three times higher"!

For the sake of that 0.07 g/nm, I'd definitely prefer the safety of all of those extra horses when I need to buck current, wind and chop. There are many times when you can't just "put up some sail" and get where you want/need to be!

[barnakiel]

Originally Posted by kmacdonald

1 HP per 750 pounds. Nothing else will work as well.

This is also my opinion. And while sailing boats want this amount of power from their sails, power boats want the equivalent out of their engines.

It is nice to agree on the basics!

b.