Different Hull Speed for sail and power?

Steve Dion · Apr 26, 2010

What prop did you get?

sailingdog · Apr 26, 2010

It could be a prop issue. If the prop is starting to cavitate above 2800 RPM, that might explain it. Or it could be that the power of the engine is causing the boat to squat or nosedive, which might slow it down as well.

Also, many boats do have an effectively longer waterline when heeled. This was one of the ways they cheated on class rules historically.

I'd finally point out that the 1.34 * sqrt LWL is a guideline and may not be completely accurate for all boats.

David in Sandusky · Apr 26, 2010

0.5 knots of current

Sounds to me like you had a half knot of current on your bow, since you are describing the sensation of hull speed - stern down, and hard to push through the bow wave. Could have been as much as a full knot.

Could your prop be cavitating?

A boat with overhangs may have a longer waterline when healed, but that would take your hull speed up higher than implied by your LWL at rest.

r.w.landau · Apr 26, 2010

Sounds like a Knot/ Mph situation. What units does your speedometer measure?
r.w.landau

Don12364 · Apr 26, 2010

Under suitable conditions the power available from sails the can easily exceed that available from the engine.
Yes, you are correct in assuming the LWL is longer when heeled and this is why hulls were (are?) given long overhangs.

sailingdog · Apr 26, 2010

This is very true... I have done 15 knots under sail, but can only do about 8 under WOT power...

Donalex said:
Under suitable conditions the power available from sails the can easily exceed that available from the engine.

Benny · Apr 26, 2010

The sails are probably putting out more power. Some sailboats increase their waterline as they heel and change the contour of the hull as it cuts through the water which may account for some increase in speed. We also have a 2GM20F (18hp) and at 13K pounds we have a hard time reaching 5 knots under power while being able to reach 7.2 knots under sail. We get hardly any benefit from heeling as these newer Hunters like to sail upright.

MoonSailer · Apr 26, 2010

Waves allow you to exceed hull speed. We were hitting 7.5 knots surfing . Our hull speed is about 7.1. We have seen 8 knots sailing. I have allways noticed that you get more speed/rpm at lower RPMs. Hull speed is one factor but other forms of drag are involved. In shallow water waves are slower.

sundried · May 25, 2010

druid's crown 28

Hi Druid;

I have had a crown 28 since 1982 - it is a love hate relationship - some very positive but also some negative aspects to this boat. But postive outway the negative.. Her name is EBONY with a black hull - cutter rigged. Did you know one of the crown 28's was apparently built yawl rigged?? I can't imagine...

Like you I have repowered with a 2gm20f yanmar. Top hull speed is 6.2 knots at about 3,200 rpm at 2,400 with a clean bottom I get about 5.5 knots and it is plenty. Under sail the boat has pegged the knot meter - more then 10 knots but to do more than hull speed the boat has to plain and so this is what we were doing - surfing downwind in the Juan De Fuca in about 35 knots of wind and steep seas. She performed well. The hull while displacement will with enough wind "get up on the step" and plain but it requires wind and is not something you can expect under power and with a 18hp.

I'd be happy to exchange further thoughts on this boat...

Cheers and fair winds
Sundried

druid said:
OK, so I finally got into some flat, relatively tideless water to test speeds for my new prop, and the results are a bit confusing. I get about 5.2 at 2300, 5.6 or so at 2500, 5.8 at 2800, and 5.8 (with a HUGE wake!) at 3300. So, looks like I've hit hull speed, eh? Problem is: 1. with the stern in the water like it was, LWL is about 25ft, so calculated hull speed should be at least 6.5 (depending on the Fudge Factor you use), and 2. I can easily hit 6.5 under sail, and have seen 7 on occasion. So why can't I even get to 6.0 under power?

Now, the only thing I can think of is that the Crown 28 has a way higher hull speed when she's heeled. Does that make sense? Am I missing something? Hull is clean, prop is new...

(stats: boat is Crown 28, LWL at rest about 23ft, beam 9ft, 6700lb. Engine is rebuilt 2GM20F (18hp) 2.36:1 trans, prop is 13x7.5 CS 3-blade)

druid

Mike Holibar · May 26, 2010

Hi Druid,
It sounds like you might still be a bit over propped. Your engine is rated maximum 18hp at 3600rpm, continuous 16 hp @ 3400 rpm and will be giving you around 14 hp at 3300 rpm, so as it is currently set up it is a 14hp engine rather than 18 hp. For the speed tests, speed through the water is what counts and you can get this accurately by doing two runs in opposite directions, add the gps speed for each run together and divide by 2 to get the average and this will be your actual speed through the water regardless of current or tide. 5.8 knots under power sounds a pretty good speed for a 28 footer. If you want to get full power from the engine you will need to take of a little pitch (always assuming the tacho is accurate) but the extra power won't make much difference to your top speed. It will help though, when motoring into strong conditions, and it will be better for the engine and fuel consumption. Best wishes,