transom drag

[kprw]

Can anyone suggest basic reading on hull form. My heavy displacement cruiser has a considerable area of transom immersed in the water and one observes similar sized sailing hulls with a clear run aft. the design speeds are similar so why the difference

 

[mortehoe]

Hull form and CE

What's your boat? (That's a need to know question.) All hullforms are designed underwater to suit what happens above water so that a Centre of Effort couple exists between the sails and the fin/rudder. What do you think that your hullspeed is? and if you managed to raise your transom what do you think that your hullspeed would increase by or to ?

 

[Billjratt]

Re: Hull form and CE

Lots of the med boats add a (sometimes quite big) sugar scoop to alleviate this problem, acquiring some extra waterline length, and, one assumes speed, but no longer meeting class measurements, and possibly spending more in marinas. Certainly a sweet exit from the water is important, as turbulance = wasted energy

 

[pheonix]

quote "Certainly a sweet exit from the water is important, as turbulance = wasted energy"
i second this statement and as i see it design speed of the hull is a theoretical maximum speed based on hull length not the actual speed that would be achieved in given conditions. the more turbulance created denies energy for creation of speed as there is a finite amount of energy to be drawn from a particular rig in given conditions, the more turbulence then less speed derived
/forums/images/graemlins/confused.gif

 

[ex-Gladys]

I'm amazed at how much water some boats appear to drag along behind them. Dignhy racing days made me sensitive to this, and shifting the crew forrard in light airs resulted in some very strange and uncomfortable sitting positions.

What about shifting some of the stowage about? does that make a difference?

PS A sugar scop can still drag if the hull surface is under water.

 

[Porthandbuoy]

Now don't laugh. Mariposa is not very big and when I placed her internal ballast in the bilges for the first time (in my ownership) I trimmed her lovely and level.
With two people in the cockpit she dragged her transom pretty bad, so I shifted a lot of ballast forrard so she sat level with about 20 stone in the cockpit. It made a big difference in her, admittedly dubious, performance.

 

[William_H]

Hello KPRW with no details on your profile I am guessing from you question that yours is a MObo not a sailing boat. (As others have assumed Sorry if I am wrong)
A motor boat is usually designed in the hull to be able to beat the hull length speed limitation by lifting the hull out of the water. That is to plane or tend to plane. To do this a mobo needs a large flat surface at the stern to sit on the water or to lift the stern out of the water. This unfortuntely means that the stern area is flat and drags in the water at lower speeds. (very ineficient)

Now a sailing boat with no real pretensions to planing or beating the hull length rule has a stern area designed for low drag. The hull bottom usually rises up so that the bottom of the transom is at water level when stationary or going slow. This gives minimum drag at low speeds (light winds and not much drive).

So the answer is if you want a motor boat but only want to use it at low speeds ie less than water line length limitation speed you use a hull shape very similar to a sail boat but with no keel. This can be driven nicely with very low engine power and is very inefficient.

olewill

 

[EdWingfield]

'This can be driven nicely with very low engine power and is very inefficient.'

Er, pardon me William

 

[kprw]

Yes, it is a heavy displacement motor boat with round bilge form (apart from cut off transom), 14 meters waterline length and 37tonnes displacement. What 'displacent' speed would be expected from this this configuration? 50 years ago some one told me 1.25 square root waterline length 8knts.... does anyone agree or disagree?

 

[oldsaltoz]

Loa 40 feet with 12 foot beam, owner wanted to reduce the noise and confused flow at the transom.

We added about 4.5 feet in length and around 6 feet below the water line to get a smoother curve, set the top of the extension about a foot below the rear deck an added a door to the transom with a step down onto the new tender come swim platform.

Reduced the turbulence and noise very well, not sure if it did anything at all to reduce fuel consumption, but owner did comment a lift in speed for the same rev's. Could be due to a much cleaner bottom when launched.

Avagoodweekend......

 

[William_H]

Hello Teredo yes you picked up my deliberate mistake. just checking to see if you are awake. Obviously the -in just jumped in there to confuse everyone.

The 40 ft round bottomed motor boat probably has a hull designed for displacement cruising ie near hull speed. The stern half of the bottom probably rises from the deepest part to near the water line at the transom. So you will find as you approach 8knots that the bow wave becomes quite large as does a wave from the stern. In fact the boat seems to dig a hole in the water from which it is trying to climb out of.
This is the effect of the hull length limited water water limit. It is not a hard limit but a speed where further power will give decreasing amount of speed increase. ie lots more power for little speed increase.

Yes 8 knots will be getting near your best speed for power. You should experiment however as different hull shapes have different amounts of hull speed effect. So a catamaran with narrow hulls has much less effect than a beamy boat.

Now back to the dragging the transom I would suggest that you try to move weight forward to lift the transom to nearer the water line. But I suspect you will find the stern squats down more at higher speeds.
This may be simply a penalty for trying to go too fast. olewill