How Fast Does Your Boat Motor?

[TrentePieds]

Quote: "Maybe I just haven't gotten the "power boat" out of my system yet"

You may well be right :-) The Cherubini27 looks like an excellent little boat for her size. She was built to SAIL. The motor was indeed only an AUXILIARY intended to take the boat into and out of her slip, and when the Cherbini was young, the marinas were smaller and simpler, so intra-marina navigation and handling didn't the require the power it does today in order to confer safety.

Remember that her length on the waterline is only 22 feet, and she is NOT a planing hull. Being a displacement boat you cannot make her go faster than 1.3(square root 22) = 6 knots however much power you put in 'er. 8HP will drive her comfortably at 4 knots (15 minutes to go a mile) if your bottom and your prop are clean. To get another knot (5 knots = 12 minutes to go a mile) you have to use 50% more power, or thereabouts, say 12 HP, and to get her to go 6 knots (10 minutes to go a mile) you'll need 150% more, say 20 HP. That is just in the nature of the hydrodynamics of boats and propellers, and you cannot change it. :-)

When newer designs have much more horsepower than did boats when Cherubini was young, it's because we so often motor rather than sail. Motoring against a stiff wind, or driving the boat through two foot waves, at those same speeds, you need about 15 HP in a boat like this. 20 is better :-)

Next time you are out, measure your speed through the water. That's what counts. Measuring against the land (or reading it off a satellite, which comes to the same thing) doesn't count. You gotta measure your speed through the water. If you do that, you can disregard the question of current.

But more importantly you will get a true indication if there is something wrong with the boat or whether you "just haven't gotten the "power boat" out of [your] system yet"

Cheers

TrentePieds

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[markpierce]

Can go up to hull speed with 80 HP on a 14-ton boat. Usually move at one knot below maximum speed with 50 percent fuel saving.

[Diesel Bill]

Last year I tried to enter Long Beach Ca. harbor south entrance during max. ebb tide. I had both Perkins engines up around 2,000 RPM for 1/2 hour and only made 2-300 ft of progress. Was like standing still. Currents can affect your forward movement to the point that you are actually going backwards. Once long ago with my old boat (27' LOA) coming into San Francisco Bay from Half Moon Bay I hit an out going tide while coming in under the G.G. Bridge. Had 25-30 knots wind funneling into the bay from dead astern with main and jib set wing and wing. I was surfing down the large swells and my flat bottom dory hull was lifting up and planing at times down the face of the waves.
If was exhilarating and scary at the same time. After a while I realized I was not making any forward progress at all. The outgoing tide was equal to my speed through the water. I ended up starting my 25 H.P. Grey Marine flat head 4 cyl. to get past the south tower. Reading the water rushing past the buoys the current was easily 5-6 Knots. As a side note a submarine surfaced heading out bound under the bridge.
That was the strangest sight. Seemed really dangerous as for shipping in the main channel. I guess they knew better what they were doing than I knew what I was doing.

[Dymaxion]

28' Viking, C&C design, 4800lbs. 8'5" beam. No prob hitting hull speed of 6.4kn with my 1GM10 at 3000-3200rpm. Folding 2 blade prop and clean undersides. Usually stay at 5kn/2800 since I'd like engine longevity. Get vibes at times, a nudge of the throttle up or down usually gets me away from the resonance. GPS speed is meaningless, need a knot meter for actual speed through water.

[TooCoys]

So today we went for a two hour afternoon sail - our second sail ever on our own boat.

According two two different gps readings, we motored around 4 knots, into the wind, with 1-2 foot chop in the bay.

We also sailed up to 5.1 knots on a beam reach with just the mainsail up!

So while my little CaySea is underpowered, I guess she does what she does ok and I just need to get used to slowing down!

[markpierce]

Did you consider currents?