1.34 x Square root of Water line length in feet will give you hull speed in knots. So about 7 knots.

 

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.

 

5-7 kn

 

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

 

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).

 

5.5 kts flat water with 1 bikini and 1 case of beer aboard...cruising.

 

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.

 

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. .....

How fast your boat is moving over the ground is meaningless.

Start with the RPMs. You said the motor is rated to max at 3600 and it should get there at wide open throttle, when underway. No matter how fast you're going. How fast you're going can be affected by many other things, but getting to max RPM is a sign of properly drive train configuration and engine health. Improper prop pitch can do harm to a motor over the long term.

While low RPM at WOT is a sign of an over pitched prop (ie there is too much drag on the blades which bog the motor down), the tachnometers on boats can be wildly wrong too. I would get a laser tachometer and confirm those settings on the engine itself. They cost under $20 on Amazon, you just put some reflective tape on a pulley and point the laser at it. Once you know the RPM is correct, then mess with the prop. +/- 100 RPM isn't too bad.

I would also highly recommend calling the manufucturer of MaxProp, PYI. They are extremely helpful and I would want to confirm you have the proper size prop in the first place. If the blade diameter is wrong, fixing it with pitch isn't the right solution, even if it's better for the engine. Who knows if the prior owner fitted it properly, or just bought a used one.

SOG may be a mistatement on your part, but it refers to speed over ground, which is measured by a GPS as your speed over the earth's surface. It's affected by current. Hull speed is measured by speed through the water. You may be making hull speed through the water, even though facing an oncoming current and seeing slower SOG. Speed isn't the issue here. Good luck troubleshooting the RPM.

 

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


A clean bottom is good.

 

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.

 

You should be able to run out to redline in calm seas and wind. That may or may not be close to hull speed. If you can only get 3000 rpm your prop is over pitched. Dialing in less pitch will get you more rpm but may not increae the speed all that much. It should allow you to maintain a higher speed in less than ideal conditions as the engine will be able to rev higher against wind and water. Personally, in a boat like yours, if I could maintain 6 k would be happy and would proably run at 5k for best economy and speed.

 

Six kts is pretty darned good for a design with a LOT of wetted surface and significant displacement.

As others have noted, keep that large area of bottom clean, very very clean. It is surprising how much a thin layer of slime will slow your boat.

 

I had been fighting RPM issues with my own boat and a yanmar 1gm10c saildrive unit. One of the things you can do with the yanmars is breifly run them up to max rpm in neutral with no harm. In fact it is one of the diagnostic tests in their service manuals. So for myself, after replacing the fuel feeder pump and rebuilding the cylinder head, I can now hit 3780 RPM in neutral and 2800 rpm in forward while tied to the dock. I was previously only getting about 2800 rpm in neutral and 2250 in gear.

 

IIRC the rule of thumb (1.3 or 1.4 times square root of waterline length) is based partly on the fact that a displacement hull will create a trough in the water as it travels. So, there's a bow wave and a stern wave and once you approach 1.4 times etc., the displacement hull has dropped into the trough it has created, so such an extent that it can never get past that, unless it has a planing hull.

Of course if you're underpowered for any reason, you won't get that far even with a tailwind.

 

As for engine HP to boat, yeah that's alot of variables (even when you take windage and sea conditions out of the equation).

My old Tanzer 7.5m (25ft~), 4000lbs~, shoal keel with a beat up old Honda 10hp outboard would do hullspeed at WOT.

My new Mirage 33 has a 2gmf (13hp) and at 2200rpm it'll hold about 5kts. I didn't want to push it more because as far as I'm concerned I'm happy anytime I can motor 5kts of speed on a boat without flogging the engine.

As far as the RPMs go, that's a great tip on the laser tach, I didn't even know those exist and I will be buying one shortly. On the 2gm engines I have heard the exhaust elbow slowly clogs with soot over the years. Some signs are black on the transom or puffs of soot when throttling up, but there may be no real sign other than loss of RPM. As the engine loses it's ability to exhale via the exhaust it also loses the ability to rev to proper RPM. It may be something to look into.

 

I’ve got 24hp with the same displacement and waterline. 6.7 knots is as fast as it goes. Cruising at 2/3 throttle gives me 6 kts.

Personally, I think yours is performing well. Finer pitch might get you more rpm, but not more speed, and will likely affect reverse adversely. So I would leave it as it is.

 

Most sailboats swat at bit more under power than under sail. Some swat a lot. Depends on shaft angle geometry of the stern( rocker, increase in submerged volume etc.). Displacement hulls really feel these effects so speed v hp is very boat specific. Swatting really slows a boat down.
Would suggest yo read some David Gerr before messing with your prop pitch. There’s a science to this and he has it pegged.
Empirically I know my boat swats under power. I know once I reach a point I get little more in the way of boat speed by increasing rpms. Just more swat. It’s real variable where this point is. Impacting variables seem to be full/near empty fuel/water tanks, sea state, direction relative to swell etc.
Sailing we look to be going high 7s/low 8s. Powering 6.5 to 7.

 

do you mean squat? If so, as the hull gets to moving hull speed it creates a wave with the length of the LWL. If you push it faster the wave length increases and the boat is then trying to power up over the bow wave with the stern dropping into the trough of the created wave. Moving faster than hull speed the boat feels like its stern is squatting.

 

A good diesel should run efficiently at 1800 to 2000 rpm regardless of speed. If your boat is set up properly it should do 5 to 6 knots in calm water at that RPM. I never run my engine higher than 2000 rpm. But then again I never have engine problems.

Diesel Mechanic

 

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

Maybe some haven't significantly re-read the initial post.

A 16 HP engine on a full keel, heavy boat is never going to push it anywhere near Hull speed.
Is it?

I would suggest it's an old boat when engines were defiantly "axillary power" not designed as main power.

My boat has a similar displacement but fin keel and it has a 57HP engine in it.

To consider his to go the same speed as mine is not looking at physics. Unless he has added something to his fuel... Maybe cocaine. :grin

 

This is something of an old thread, but Mark is right, this is too small an engine to push this boat to hullspeed. My boat weighs virtually the same weight as the OP's boat, and probably has the same or less wetted surface. It has an easily driven hull form that is optimized for reaching speeds which is similar to a semi-displacement hull form that is optimized for powering. I have the three cylinder version of the same engine (a 3GM30F, rather than the 2 cyl in the O.P's I.P.). It pushes my boat at around 6.3 knots in flat water at around 2,900 rpm cruising speed, which is substantially less my boat's 7.6 knot hull speed. Even wound up to 3,200 rpm, I only get into the 6.8-6.9 knot range. Arguably my prop could be pitched a little more aggressively and have a little more area, but even so, I seriously doubt that I would get much over 7 knots since the engine seems to be self limiting around 3500 rpm which is below the point that the mechanical rpm limiter kicks in, which suggests that at 3,500 speed is limited by HP and not the prop.

Jeff