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Old 09-11-2022, 13:21   #1
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How much oohmp (torque?) should I expect from a sailboat?

I have a 2GMF with 13hp and fixed, 2 blade prop. Whole powertrain has performed supremely thus far. I was supposed to have it hauled out today, but couldn't get out of my slip. My slip is a floating dock on the S side with the exit on the E side. Wind was blowing SSW at about 20mph plus gusts- blowing me into the slip and hard into the side floating dock. I had a heck of a time pulling enough slack into the N dock lines to loose them. At the time, the down wind docks were clear, so I wasn't worried about wrecking into other boats, else I wouldn't have attempted. I ended up at full throttle, backwash was as expected, but the boat was glued to the dock.


So the question is: how much oomph can expect out of my powertrain? I Imagine that there was a tremendous amount of friction holding me there, but I now wonder if I could have motored thru that wind. I may go out on a calm day and try to get an idea of lbs of thrust I'm producing.
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Old 09-11-2022, 13:28   #2
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Re: How much oohmp (torque?) should I expect from a sailboat?

Sounds like a prop issue. If the engine was turning the prop and you weren't moving it means the engine had enough torque but the prop couldn't grab the water enough. My boat is a little shorter, but not much, and I have a 14 HP diesel - about the same as you, but I have three blades on my prop and have never had that sort of problem, even in strong wind.
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Old 09-11-2022, 13:29   #3
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pirate Re: How much oohmp (torque?) should I expect from a sailboat?

I would say on your boat ideally you need a 27hp engine, what you have at the moment would be struggling to reach 2-3kts in a 20kt headwind.
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Old 09-11-2022, 13:45   #4
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Re: How much oohmp (torque?) should I expect from a sailboat?

My first thought is how clean is the prop ? Even just a few barnacles really take the edge off your performance. Do you know that it's clean ?
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Old 09-11-2022, 13:53   #5
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Re: How much oohmp (torque?) should I expect from a sailboat?

Quote:
Originally Posted by boatman61 View Post
I would say on your boat ideally you need a 27hp engine, what you have at the moment would be struggling to reach 2-3kts in a 20kt headwind.
I can believe that. In zero wind and calm water, I get 5.5 kt.

Quote:
Originally Posted by Brioche View Post
Sounds like a prop issue. If the engine was turning the prop and you weren't moving it means the engine had enough torque but the prop couldn't grab the water enough. My boat is a little shorter, but not much, and I have a 14 HP diesel - about the same as you, but I have three blades on my prop and have never had that sort of problem, even in strong wind.
Powertrain is on the list of things for them to check out. Wish I could give more info. I keep forgetting to check what my gear ratio is. According to marinedieseldirect.com, the 2GM has a 2.6:1 ratio and recommends:
2 blade: 16x9 or 15x11
3 blade: 15x9 or 14x11
for reference, they say the 3GM30 (27hp, 2.61:1) 17x12(2 blade) or 17x11(3 blade)

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Originally Posted by OldManMirage View Post
My first thought is how clean is the prop ? Even just a few barnacles really take the edge off your performance. Do you know that it's clean ?
It's not got big growth, but it's definitely not optimal. The whole bottom had heavy growth when I bought it and they did a good bottom scrape. Bottom paint is the main reason it's going in.
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Old 09-11-2022, 13:59   #6
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Re: How much oohmp (torque?) should I expect from a sailboat?

That's quite a big prop and with a fouled hull and small engine that's it. After cleaning the hull and prop do have a good look to see what size prop is actually fitted to the boat. The when back in the water see if it can achieve full revs in fwd with a clean hull and normal load of fuel and water etc. If not its too big a prop or too small and engine and you need to change the prop.

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Old 09-11-2022, 14:18   #7
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Re: How much oohmp (torque?) should I expect from a sailboat?

Don't assume you have that gear ratio as there are several variants for those transmissions (mine is 2.21). There is a rating plate on the stbd side of the transmission case. Hold your phone down there and take a snap and verify the actual ratio. And yes, you are underpowered. The 2GM in my 3.3 tonne 28 footer is just adequate with a 3 blade prop.

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Old 09-11-2022, 14:47   #8
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Re: How much oohmp (torque?) should I expect from a sailboat?

I have 14hp in my O'Day 30 driving a 2 blade fixed prop.

I've not wanted (or needed) more power. Even motoring into 40 kt winds, I've not needed WOT. Docking and leaving the dock I've probably never used more than 50% throttle in reverse.
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Old 10-11-2022, 09:55   #9
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Re: How much oohmp (torque?) should I expect from a sailboat?

au chante:

The "rule of thumb" horsepower requirements for a small boat such as yours [and mine :-)] is:

A) For "in and out" of an "easy" marina characterized by little wind and no current to speak of: 2 hp per ton displacement.

B) For getting from "A" to "B" over short to moderate distances in benign weather: 3 HP per ton

C) for getting from A" to "B" over longer distances or in weather characterized by strongish headwinds and moderate chop: 4HP per ton

Always assuming that the engine drive a prop properly dimensions for the boat/engine combination.

Your boat, like TP, will displace 11K lbs laden, i.e 5 tons. You have 13 HP, i.e. not quite 3HP per ton, which is marginal, and would be too little for me in my waters. Therefore, I have 20HP, i.e. optimum bang for the buck in terms of engine acquisition cost, engine maintenance cost and fuel consumption.

TP is of similar hull form to your boat, so the prop requirement should be similar for the two boats. I have a 16"D x 12"P 3-bladed prop which serves me well because being a "pilot house" boat, TP is sort of a motor sailer

Go here:

https://vicprop.com/displacement_size_new.php

and put in your boat's parameters. The calculator will tell you what prop you SHOULD have. Then, when your prop hub is clean so you can see what is stamped on it, you will know if you are close to optimum. Your prop may be the wrong specification, and in any event, you were attempting to make sternway against a fairly strong wind. Props are FAR less efficient when turning in reverse than they are when turning the "proper" way - and that is little enuff :-)! When in reverse the ogive (the curve of the blade section) faces the wrong way! So if your prop is foul, I'm not surprised that you got nowhere.

That the hull might be foul would make no difference in that particular situation and at that particular time since you weren't moving the boat through the water.

Play with different parameters in the prop calculator to gain an intuitive feel for how the different parameters interact with each other to determine optimum diameter and pitch of the prop. :-)

All the best

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Old 10-11-2022, 10:03   #10
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Re: How much oohmp (torque?) should I expect from a sailboat?

Quote:
Originally Posted by TrentePieds View Post
au chante:

The "rule of thumb" horsepower requirements for a small boat such as yours [and mine :-)] is:

A) For "in and out" of an "easy" marina characterized by little wind and no current to speak of: 2 hp per ton displacement.

B) For getting from "A" to "B" over short to moderate distances in benign weather: 3 HP per ton

C) for getting from A" to "B" over longer distances or in weather characterized by strongish headwinds and moderate chop: 4HP per ton

Always assuming that the engine drive a prop properly dimensions for the boat/engine combination.

Your boat, like TP, will displace 11K lbs laden, i.e 5 tons. You have 13 HP, i.e. not quite 3HP per ton, which is marginal, and would be too little for me in my waters. Therefore, I have 20HP, i.e. optimum bang for the buck in terms of engine acquisition cost, engine maintenance cost and fuel consumption.

TP is of similar hull form to your boat, so the prop requirement should be similar for the two boats. I have a 16"D x 12"P 3-bladed prop which serves me well because being a "pilot house" boat, TP is sort of a motor sailer

Go here:

https://vicprop.com/displacement_size_new.php

and put in your boat's parameters. The calculator will tell you what prop you SHOULD have. Then, when your prop hub is clean so you can see what is stamped on it, you will know if you are close to optimum. Your prop may be the wrong specification, and in any event, you were attempting to make sternway against a fairly strong wind. Props are FAR less efficient when turning in reverse than they are when turning the "proper" way - and that is little enuff :-)! When in reverse the ogive (the curve of the blade section) faces the wrong way! So if your prop is foul, I'm not surprised that you got nowhere.

That the hull might be foul would make no difference in that particular situation and at that particular time since you weren't moving the boat through the water.

Play with different parameters in the prop calculator to gain an intuitive feel for how the different parameters interact with each other to determine optimum diameter and pitch of the prop. :-)

All the best

TrentePieds
Awesome info! Thanks!

And note that I back into my slip so I was getting max performance. Now that I think of docking the boat, I’m not too surprised I didn’t make it out. When moving forward, emergency stops are non existent. When backing in, I’ll be moving just fast enough to get rudder athority… which is slow walking speed. Even then, stopping the boat by putting in forward takes forethought.
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Old 10-11-2022, 18:19   #11
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Re: How much oohmp (torque?) should I expect from a sailboat?

I assume the end of the prop shaft is tapered so prop can’t be put on backwards? You write like you have good reverse power which is not normal, most props are optimized for one direction.
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Old 10-11-2022, 18:21   #12
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Re: How much oohmp (torque?) should I expect from a sailboat?

Looks right in the bottom photo though.
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Old 10-11-2022, 19:26   #13
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Re: How much oohmp (torque?) should I expect from a sailboat?

What you are talking about is reverse static thrust (engine in reverse, boat attempting to move backwards, but not moving) sometimes called reverse bollard pull. In the US it might be measured in pounds force. In the rest of the world in kilograms force or more properly in Newtons.

Take a look at https://www.yachtingmonthly.com/gear...ler-test-29807 . They had a 21 hp (vs your 16 hp) at 3600 rpm Yanmar driving the propeller through a clutch with a 2.6:1 forward and 3.0:1 reverse ratio. (The propeller turns slower in revers than in forward at any particular engine speed. It is normal for transmissions to have different ratios in forward and reverse. You will have to look at your transmission as three different pairs of ratios were available.) YM tried several propellers. On reverse bollard pull there was a 2X range of thrust in depending on the type of propeller fitted. Two blade propellers were generally worse than three blade.
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Old 10-11-2022, 19:32   #14
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Re: How much oohmp (torque?) should I expect from a sailboat?

OP backs into his berth so he was having problems getting out in forward.
We’re trying to figure out why.
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Old 11-11-2022, 00:10   #15
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Re: How much oohmp (torque?) should I expect from a sailboat?

14hp should be plenty around the docks.

In 99% of conditions, you rarely do more than the slightest goosing of the throttle docking unless things going badly wrong, so giving it full throttle should be more than enough to drag her along the dock (might cause some scrapes but it should move).

Where 14hp is likely to come up short is trying to motor at speed into a current or headwind.

The hull growth shouldn't matter much for docking unless really excessive. It really doesn't have much impact until you get the boat moving thru the water...in which case, you wouldn't have posted your question.

The prop growth very much can have a big impact and with a 2 blade prop that has less grip on the water, it can compound the problem. While the picture doesn't look horrible, a surprisingly small amount of growth can substantially reduce the prop's efficiency. Next time you are out, see if you can dive with a green scrubby and clean it up...then check how it performs.

When you get out to open water, how fast does she go when you give her full throttle? What RPM does that correspond to? For a low powered 31ft boat, I would expect something on the order of 6-7kts in calm conditions and the motor should be at or around max RPM once it gets up to speed. If you are struggling at 4kt and 1000RPM below max, there is a problem.
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