boat squatting under power

[catalac08]

Hi
A sailboat under power query so I guess that here is where the expertise is!
Stern squats badly under moderate power (boat 27ft cat with 30 hp outboard), about 6" down.
My query is what amount of power is being wasted by forcing the stern down like that and whether it is worth modifying the transom as there is no further adjustment on the trim positions. This is reasonably easy to do but I need to have some belief it is worth spending a day doing this.
Also I have read that just having the prop deeper can lose 200rpm/extra inch of depth and I would be interested to hear any views on this.
The engine is probably overpropped anyway and I probably need a finer pitched prop suitable for a displacement boat but I think this is a seperate issue, but not certain.
I appreciate that lifting the bows is not all bad as the last thing you want to do is drive the bows under but the bows have massive flare/reserve buoyancy and when sailing the boat is trimmed much more level without problems
Thanks for any views/ideas.

 

[snooks]

Nearly all yachts will "squat" down when motoring to a certain extent, and it becomes more apparent the harder you're motoring

As I understand it, and I might well be wrong, it's the angle and position of the prop shaft pushing the bow up and the stern down. The centre of effort under power is different from that under sail so the hull goes through the water slightly differently.

Older yachts with finer transoms and therefore less buoyancy, like mine, squat a little more than most (the same could be true with your cat), a colleague with a Contessa 32 has to take a step forward in the cockpit when he's motoring hard, as the water start coming back through his cockpit drains, and he gets wet feet!

 

[phockit]

How fast are you going at "moderate throttle" ?? Seen similar boats with just 10hp on the back so are you starting to exceed the displacement speed of the boat and therefore trying to drive it up the bow wave(s) ??

Paul

 

[Tranona]

The squatting is because you are exceeding the hull speed of the boat. That is you are at the point where you are falling into the trough of the wave produced by your movement through the water. This happens on many boats, usually because like yours they are overpowered in relation to that required to achieve hull speed. Any additional power results in squatting and little additional speed.

Simplest thing is to not use full power, or reduce the amount of thrust in relation to the engine revs by a finer pitch prop. The real solution is to have an engine appropriate to your boat, and I guess that 15hp would be more than enough to achieve hull speed, but probably 20 to give reserve would be acceptable.

 

[catalac08]

Hi
Thanks for rapid response with some ideas.
I am not referring to speeds in excess of hull speed. At full throttle the engine will only achieve 6.5 knots and the squatting is bad at 5 knots. The hull speed depending what factor you use is about 7knots for the 25ft waterline length so nowhere near to that. Yes many catalac 8m do use 10 hp engines, albeit the Yamaha Hi thrust engine which as a 2.4:1 gear and a relatively big prop for that engine size and with an ultra long leg is probably the best engine overall for a cat. My reason for a bigger engine is really to be able to motor sail with strong headwinds and still make some progress to windward, albeit slowly. With the smaller engine that can be quite a problem.
Looks like some experimentation with the transom and a fine pitch prop unless there are any really inspired posts on this problem.

 

[Tranona]

If you are not achieving hull speed with that horsepower then there is something seriously wrong with the prop/gear ratio. The squatting may be due to excess weight at the stern that is not supported by the bouyant sections, or it could be the angle of the outboard. You may find moving weight forward helps. However a 30hp engine is probably twice the weight of a 15, so just reducing the size/weight of the engine will help.

 

[Malcb]

Check the trim angle of the Outboard.

The horizontal 'peg' which clamps the out board when in reverse could be in the wrong hole. There are usually 4 positions which will give various vertical positions of the O/B leg.

 

[catalac08]

hi thanks for all that:-
my plan is to
1 get a finer pitch prop & try and see max rpm/speed under power
2 pack out transom at rear side to get a better angle for the prop to bring up stern (no more adjustment on holes/peg - measure squat. Repeat until only say1/2 inches squat.

Some suggestions to redistribute weight forward - I dont think this will work as then when sailing I will be bows down as the boat is level when stationary now.

The more I think about it the more convinced I am that squatting 6 inches with the increased rudder resistance (2) and increased leg immersion apart from the increased wetted area of the hull and the flat transoms immersed will slow down the boat substantially.

Another thought is to use Doel type fins on the leg - some people swear by them - I suppose some of the thrust going upward could be deflected to the rear and result in some lift and more thrust going to the rear giving drive?
I live in hope......

 

[Rum_Pirate]

[ QUOTE ]

2 pack out transom at rear side to get a better angle for the prop to bring up stern (no more adjustment on holes/peg - measure squat. Repeat until only say1/2 inches squat.

Some suggestions to redistribute weight forward - I dont think this will work as then when sailing I will be bows down as the boat is level when stationary now.


Another thought is to use Doel type fins on the leg - some people swear by them - I suppose some of the thrust going upward could be deflected to the rear and result in some lift and more thrust going to the rear giving drive?
I live in hope......

[/ QUOTE ]

I too recommend the checking of the trim angle of the Outboard.
The horizontal 'peg' which clamps the out board when in reverse could be in the wrong hole. There are usually 4 positions which will give various vertical positions of the O/B leg.

I suspect that you are trying to pwer the boat to go too fast. The engine is then trying to get your boat over the hump and ''on the plane' but does not have the power (Waste of time and money to try) as the hull shape for planing is wrong.

Doel type fins on the leg may help a little but will not solve the problem entirely. These fins help boats get on the plane and once there run at lower throttle settings.

Throttle back/get a lower powered engine/get a finer (less pitch) prop and take life a little slower.

 

[Bajansailor]

Just my two cents worth re hull speed - I think that hull speed in the conventional sense of approx 1.34 x square root of waterline length applies only for 'conventional' shaped hulls, ie those with a length to beam ratio of approx 3.
Catamaran hulls will generally have a much higher L/B ratio - if the waterline beam of your Catalac is say 4', and the WL length is say 24', then the L/B ratio is 6.
Hence the max theoretical hull speed for a cat should usually be greater than that prescribed in the formula, as the hulls are much thinner.
Yet your vessel is squatting long before she reaches hull speed, even when using the conventional formula.
Is she heavily loaded, to below her design waterline?
Perhaps look again at trim / longitudinal weight distribution?

There is a Catalac 8m. here which is used as a motor boat only - the owner took the rig off her some years ago. She has a couple of inboard Volvos with sail drive units (I am guessing that they are probably 2002s, about 18 hp each) and she scoots along very happily with this configuration.

 

[Tranona]

There are plenty of outboard powered Catalacs around and those that I have seen seem to perform well under power. Have you tried the owners association?