SMJ Minus

wml8435 · Post by **wml8435** » Tue May 09, 2006 5:28 pm

Will the SMJ Minus get a 12 foot boat up to hull speed?

Post by **John@sos** » Wed May 10, 2006 10:19 pm

Hello there, sorry I don't think there is enough go in a Forty minus, it is only 2 hp max. you should be looking for a FP model maybe,

Regards,
John
SOS

CatiGull · Post by **CatiGull** » Thu May 11, 2006 2:32 am

John - my Forty Featherweights take my Island 17 up to 5 knots....do I have a supercharger on these Im missing or is the Minus less powerful??

Post by **John@sos** » Thu May 11, 2006 10:33 am

As the Featherweight is just 1 to 2 hp max and the power needed normally with displacement craft can be equated to the following formula:

WL length in metres = the power needed in bhp = to the speed measured in Knots... This works for all displacement craft up to 30 ft long.

If you wish to work out the design speed for the hull it is possible by taking the square root of the waterline length in feet and mulitplying it by 1.4. This formula works for all displacement craft, the 1.4 is sometimes changed to 1.3 or 1.5 depending on special hull shapes, but 1.4 is the average. It is a law of hydrodynamics. To go faster you have to plane....

A 12 ft long dinghy will probably be less than 4 metres on the Waterline, but assuming it is 4 metres.....

I.e. 4metre long boat needs 4 hp and will get 4 knots. You cannot get more speed out by increasing power above 4 as the WL length is the speed liniting part of the formula. the power needed normally has to roughly equal that .

If you are really getting 5 knots from a less than 2 hp motor you should enter races!

The dinghy must be very slippery! At least you can save weight and probablty fuel by using the smaller motor.

Good luck to you.

Regards,
John
SOS

erik0905 · Post by **erik0905** » Thu May 11, 2006 1:35 pm

CatiGull wrote:John - my Forty Featherweights take my Island 17 up to 5 knots....do I have a supercharger on these Im missing or is the Minus less powerful??

It is not the overall lengt of your boat, but waterline lengt it's about, and perhaps you can place yourself in the front and lift the rear, and get a shorter wl. which doesn't need so much power, we are op against nature.
There must be an explanation otherwise, yourself and John are far apart in this matter.

CatiGull · Post by **CatiGull** » Sat May 13, 2006 11:16 am

Ive measured the speeds with GPS so unless Ive mucked up the satellites im sure on the numbers.

My Island is a very flat bottomed dinghy which the hull speed equations probably underestimate the speed.

Eriks note about using waterline length is correct of course, and that shifting weight in a small boat alters these numbers.

My Featherweight is working at full throttle to hit these speeds, the Forty Plus is not working as hard.

Now to find some cash 'Gull races on the Hudson...

Still working on the EFNR..........

CatiGull

On edit: Im sure Im doing better than the 2 kts of the rule above, John - the Hudson runs a good 2-3 kt tide and Ive taken the Island up against this tide many times at 2 kts or so....must be the hull shape is fast

It is a wonderful boat that needs but a breath of air to move...