Increasing running trim

[Ben Land]

Dear all,

I was wondering if there is a system to lift planing boat bow up = increase running trim angle?

Trim tabs, transom flaps, hooks, transom wedges and interceptors can be installed to increase bow down moment but I am looking for system to do the opposite.

Please consider a case where moving center of gravity further aft is not possible.

This is a craft of approximately 8m lwl and top speed is currently 37.5kn which can be increased to 39 easily by adding ballast weight in aft. However this is not accepted as permanent solution.

According to literature, extending lifting strakes all the way to transom might move the center of pressure more forward decreasing bow down moment. The owner might be willing to try this but first I want to exhaust all other options.

I appreciate all ideas.

BR
Ben

 

[Mr Efficiency]

Is this a shaft drive ? You have not given a lot of detail, deadrise angle is what ? Any pictures ? I'd be surprised if extending strakes further back would help at all, more likely hinder.

 

[gonzo]

Lifting strakes, as their name indicates, lift. Extending them aft, will increase lift at the transom. Adding a strake forward may help, but will likely cause dynamic problems like porpoising. Seems like the solution is shifting weights. If you take one Kg forward of the CG and add it aft the same distance, it is the equivalent of adding two Kg aft. However, it will change the trim without adding weight and increasing resistance.

 

[Ben Land]

Thanks for the replies,

This particular case is a waterjet drive with shaft inclination angle of 4deg relative to keel.

I am not looking a solution for any particular case but just wondering what are the options in general for this size / speed range. Values given are just an example.

Lifting strakes also known as spray rails also detach spray and Water from hull making the wetted beam smaller in step. It was given in Principles of Yacht Design that in cases where boat approaches 40 knots in speed, the lifting strakes in transom area move the center of pressure forward due to narrower wetted Surface area in transom. I think this is interesting theory but migh be too generalized as the lifting strakes shape, size boat deadrise etc must play part as well…

Gonzo, makes sense regarding playing around with weight distribution but I am trying to find another solution as this is not preferred solution in many cases.

Ben

 

[Ben Land]

Lets assume a deep-v deadrise of 20-25degrees, monohedreon hull.

 

[DCockey]

It was given in Principles of Yacht Design that in cases where boat approaches 40 knots in speed, the lifting strakes in transom area move the center of pressure forward due to narrower wetted Surface area in transom. I think this is interesting theory but migh be too generalized as the lifting strakes shape, size boat deadrise etc must play part as well…

Lifting strakes will change the pressure distribution on the bottom addition to the wetted surface area. Both changes must be considered in evaluating the effect on boat trim.

 

[Barry]

The easiest way would be is to install a jet-o-vator (or equivalent) or shims/wedges at the nozzle.
The advantage of a controllable nozzle is that if at higher speeds you want to suppress porpoising you can deal with it.
Alternatively, you can keep the nozzle down for acceleration and if for a period of time, you want to max speed, you can put the nozzle even higher
up than say a normal angle. (barring some instability and steering issues that may develop) ,

 

[Mr Efficiency]

News to me that extending strakes back would decrease lift aft, but it does appear you best solution is one that will allow the jet nozzle to direct water more upward.

 

[Barry]

News to me that extending strakes back would decrease lift aft,

News to myself as well.
Ben makes the comment in an earlier post "Lifting strakes also known as spray rails also detach spray"
I disagree a bit on the terminology and offer perhaps a different perspective
Where the boat lifting strake is under water, ie is not ventilated to atmosphere, there is no spray that occurs. The lifting strake provides additional lift
due to the change in direction of the water. If the hull is a constant 22 degrees of deadrise the water would move to the chine without any change of direction. With the addition of a 'horizontal lift strake' which then changes the water direction to horizontal, lift is produced. Force = Mass (flowrate) times acceleration. Acceleration is a change in velocity. Velocity has two components, speed and direction, so changing direction creates an acceleration and hence a resultant force.

So I would call a lift strake a spray rail only on the portion where the water exits the strake to atmosphere but if water is coming off of it, it will produce lift

While the outside chine will separate spray from the boat but also provides lift as a lift strake, even though the water spray is going to atmosphere.

If your statement that a lift strake reduces lift as it goes to the transom , it would then follow that the strake will reduce lift over its entire length. The water movement would be much the same from just behind the stagnation point at the front of the hull until the back of the hull. ( making the assumption for simplicities sake that the hull portion in the water is only the monohedron portion

It has been our experience that by modifying the shape of the strake, we got more lift and we always ran the strakes to the back.

This is a very simple description and does not include additional lift from the flat portions of the lift strake due to it degree of orientation to horizontal
As do most of the boat manufacturers

 

[Mr Efficiency]

If there is a substantial flat at the chine aft, and this an alloy boat and it can reasonably easily be truncated a couple of metres short of the transom, that will increase the running trim angle, but also create more bank in turns, and a slower rise onto plane. But it may be structurally too difficult.

 

[Ben Land]

Thanks everybody for your replies.

I disagree a bit on the terminology and offer perhaps a different perspective

Barry I agree with you on the terminology, need to be careful with the terms..

Just to make it clear it is not my statement that running the strakes all the way to the transom will reduce lift, this is just an interesting theory that I read and wanted to check other peoples opinions on the matter.

 

[BlueBell]

You could mount a huge wing forward creating lift the faster you go!
You could pump water into a ballast tank well aft.
You could move the motor aft... oh, no, no modifications to move weight around, right.
Load your passengers aft.
Mount a downforce wing aft in the water.

But why?

 

[pacblue]

If you grind a little rocker (minus 1/4” to 1/2’) in the last foot of the bottom section and then set up your trim tab equal to that new trailing edge (zero position) you will have created a situation that will increase running trim at the zero tab position.

You will then have to use tab underway to control running trim values but have the ability to set it back to zero position and increase trim underway. Should work for interceptors as well.

 

[Mr Efficiency]

I don't think grinding the bottom would do much other than destroy the boat.

 

[pacblue]

It is called tuning your hull, performance guys have been doing it since the 70’s, adding hook or rocker, or taking it off.

It can be done in steps, starting at an 1/8” , don’t knock it until you have tried it. I have used it to fix a few production boats that ran too flat or too bow high.

This assumes your bottom thickness at the transom is sufficient to lose the amount you finally take off, but most frp boats have plenty of overlap at the transom / bottom intersection.