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post #1 of 22 Old 03-21-2018 Thread Starter
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How Fast Should My Boat Go Under Diesel

Is there a formula used to determine the theoretical boat speed under auxiliary power based on engine HP, Displacement, and LWL?

Engine - Yanmar 2GM20F
HP - 16
Boat - Island Packet 29
LWL - 27.58
Beam - 10.83
Displacement - 10,900 (empty)

The engine is rated at 3600 RPM (3400 continuous). I'm working through some engine issues and would like to know if the engine can reach specified RPM's under load, the prop is clean, properly sized and pitched, with a clean bottom, what would my expected SOG be in placid waters under auxiliary power?


Thanks,
Joe

Joe Mullee
1991 Island Packet 29
Shady Side, MD
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post #2 of 22 Old 03-21-2018
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Re: How Fast Should My Boat Go Under Diesel

1.34 x Square root of Water line length in feet will give you hull speed in knots. So about 7 knots.
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Re: How Fast Should My Boat Go Under Diesel

That would be a long formula if there was one . to many variables but comes down to HP, wetted surface and how was the prop pitched? properly pitched for what, speed or acceleration or backing up? salt or fresh water ? hot or cold day? and the Island Packet slowness factor. lots of wetted surface on the long deep full keel. all that calculates out to a not very fast 5 knots give or take a little. and a little better if you are going down hill.
The best way to tell if you are performing up to spec is to compare with another like boat with the same engine and prop.

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Re: How Fast Should My Boat Go Under Diesel

5-7 kn
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Re: How Fast Should My Boat Go Under Diesel

I would doubt it would get anywhere near Hull speed on a 16hp engine.

But I could be wrong.

This would be a speed trial type thing, not a formulea

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Re: How Fast Should My Boat Go Under Diesel

Hull speed is the theoretical top-end speed on flat, still water. You may or may not be able to achieve hull speed, depending on your engine's horsepower, prop size/pitch, gross weight of the boat and its contents, and amount of growth on the bottom. Without experimenting with your boat, nobody will be able to tell you how fast you should go.

I think the question you should be interested in is: How hard should I run my engine? Consult the engine owner's manual for that. If you bought the boat used and don't have a user's manual, a good rule of thumb is to cruise at 80% of your RPMs at maximum throttle. Bring the engine up to full throttle, check the RPMs and speed through the water, then throttle back to 80% of your max RPMs and re-check your speed through the water (rather than your GPS speed over ground because that will be affected by currents). If you reach hull speed before hitting full throttle, then there isn't any reason to run the engine all-out. And as your speed increases toward hull speed, the amount of additional power needed to make the boat go faster increases. To put this in financial terms, it might cost you $3/hr to cruise at 5 knots and $5 or $6 an hour to go 6 knots (i.e., a 100% increase in fuel costs for only a 20% increase in speed). These numbers are just for illustrative purposes; your actual numbers will be different but will likely follow a similar pattern.

Pay attention to the relationship between RPMs and speed. If you notice an unexpected drop in speed, ask yourself if you are bucking a current, have added weight to the boat, or if it is time to clean the bottom. (BTW, the slightest amount of growth on your prop will ruin its efficiency).
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Re: How Fast Should My Boat Go Under Diesel

5.5 kts flat water with 1 bikini and 1 case of beer aboard...cruising.
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Re: How Fast Should My Boat Go Under Diesel

Thank you for the quick responses.

In flat water at slack tide with calm winds I can go 6.0 knots SOG at 3000 RPM. Currently that's as high as I can get the engine to run under load (panel tach and handheld tach give the same reading). I have a Max Prop which I'm unsure of the pitch setting as i write this. When I haul to paint the bottom this spring I will check the prop.

Thanks again for the help.

Joe Mullee
1991 Island Packet 29
Shady Side, MD
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Re: How Fast Should My Boat Go Under Diesel

That's terminal velocity for an Island Packet.... Even when dropped off a crane.


A clean bottom is good.
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Re: How Fast Should My Boat Go Under Diesel

Changing the max pitch down will allow a higher RPM .This may allow the engine to produce more power and hence more speed. Figure what RPM you want at maximum and throttle back to comfortable fuel consumption. and accept it. . Sometimes it's helpful to have some in reserve for adverse current or bucking but you don't want to run like that all the time.
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