Expected boat speed

[Shooter II]

The theoretical max hull speed for my H31 is 6.83.
I've got a Yanmar 2GM20 - 2 blade
I was out on the bay this weekend and I was only able to get to about 5.4kts on the Yanmar taching about 3500rpm - about 4.8 @ 2800.

My question is, is this reasonable or is it something I should be concerned by?

thanks in advance.

Shooter ii

 

[Snotter]

Seems kinda slow. What does an H31 displace? What is the condition of your boat's bottom? What are the dimmensions of your prop?

 

[higgs]

It seems a bit slow to me as well. I wonder if that 2 cylinder might leave you a little underpowered. Certainly a two blade prop will be slower, but then a three blade will slow you down under sail, unless you go folding or feathering.

 

[Don12364]

And how was the speed measured? Log (accuracy or calibration) or GPS? SOG (tidal current) or thru water?

 

[Roger Long]

If you want a good answer, horsepower at that RPM, waterline length, and displacement would be helpful. Or, do you expect people to spend 1/4 - 1/2 hour looking that stuff up just to help you out?

 

[Grateful]

I suggest you put your mask and flippers on and go for a swim. I lose about 1 - 1.5 kts just from the slime build up on my boat

 

[Bill1565]

I'm with Grateful. Dirty bottoms and growths on your prop will eventually reduce you to one or two knots thru the water. Yours isn't that bad yet.

 

[Shooter II]

Roger,

I was just asking sailors, who may include H31/2gm20/2 blade owners, if the speed, over ground, was "reasonable". I slept on it and realized that the question I should have asked would have been about the "through water speed". Unfortunately, don't know that.
I believe the hp @ 3600 is 18-20 (depends on who rates it), lwl = 26ft, displacement = 9700.
thanks again guys

 

[Jumpstart]

Shooter, you should be able to cruise all day at 6+ knots at about 2800RPM. As others have suggested there is something else at play. Fouled prop, slimy or barnacle encrusted bottom? Start eliminating the variables. Check with a prop company to make sure you have the right prop size and pitch. Check your knot meter versus gps to see the effects of current and waves. How fast does your boat sail on a reach in 15knots?

 

[Shooter II]

thanks, guys.
I asked because I thought it might be slow and suspected the bottom might need cleaning.

 

[MrBee]

i have a 2GM20F in an Irwin 10/4, 7000lb , Wide open I don't even get that high of RPM, maybe 2800 - 2900. I can't tell you through water speed, just SOG. I can top out at about 6, maybe alittle more with the tide. Easily hold 5 against tide. (upper Chesapeake). By August and September I will loose .5 - 1.0 knots due to bottom Slime.
2 blade prop but have no idea what the measurements are. my best crusing RPM seems to be around 1800. I just wind her up till she's running smooth.

Brian

 

[Roger Long]

I believe the hp @ 3600 is 18-20 (depends on who rates it), lwl = 26ft, displacement = 9700.

My 13,000 pound boat with the same waterline length goes that speed with just 8 hp. I think it's more than slime. Sounds like your prop doesn't have enough pitch. If so, your engine will suffer eventually running at that light load.

Adding pitch won't increase your sailing drag. Actually, it will reduce it (infinite pitch is a feathered prop). You would do well to talk to a prop shop and have calculations run. You could end up a bit faster under sail as well as power.

One of the first things they will ask is to check your tach. You can buy or rent one that reads a piece of reflective tape on the shaft coupling pretty cheaply. You'll need to find out your transmission reduction ratio. It should be on the data plate or you can look it up from the model number on the data plate.

Run your GPS speed in one direction, then turn and run it in the other. The average will be close enough to your water speed for these purposes. Just be sure to let the boat come up to full speed after the turn. It can take longer than you think.

If your engine comes up to full RPM very easily with travel left over in the throttle lever and no change in exhaust smoke color, it is almost certainly too little pitch on the prop. You may not be able to get a prop with enough pitch in which case you'll need to add diameter or go to a 3 blade.

 

[Bob16841]

Roger,

I was just asking sailors, who may include H31/2gm20/2 blade owners, if the speed, over ground, was "reasonable". I slept on it and realized that the question I should have asked would have been about the "through water speed". Unfortunately, don't know that.
I believe the hp @ 3600 is 18-20 (depends on who rates it), lwl = 26ft, displacement = 9700.
thanks again guys

SOG means nothing, it's through water speed you need to check. SOG is affected by tides, wind, etc.

 

[Ross]

If your propellor isn't absorbing the horse power available from the engine you are wasting a perfectly good engine and could get the same result from a smaller engine.

 

[TR6]

I have the same boat. With a clean bottom and in flat water I get 6.5 kts through the water (Kiwipriop). It was about the same with the original 2 blade. Add some wave action and growth and the speed drops. Powering into 10 kts and 3 foot close waves drops the speed down by 1.5 to 2 knots.

 

[Joe]

thanks, guys.
I asked because I thought it might be slow and suspected the bottom might need cleaning.

Shooter... if that's what it takes for you to suspect a foul bottom... then it's really gonna scare you when you get under the boat. Have the bottom cleaned, then check your speed through the water.. or use the gps but average the speed on reciprocal courses.