Hydrasport Porpoising

[CROW]

I have a 2005 24' Hydrasport center console w/twin 150 motors. The boat has a serious porpoising problem. I like to fish off shore in the Ocean & feel it has become unsafe to take more than a few miles out. What can I do to fix this problem? I have heard that this is a common problem with this brand of boat. The dealership where I purchased the boat has been unable to fix the problem.

[KCook]

I don't have the answer. But you will find some more Hydra-Sports owners over on www.TheHullTruth.com

Kelly Cook

[Ike]

Porpoising is almost always due to one of three causes; hull shape in the aft planing surface, distribution of weight in the boat and position of the engines.

The first thing to do is check for a hook or hog in the hull in the rear quarter of the boat. with the boat on a trailer put a straight edge on the bottom (pointed for and aft) The surface should be straight with no curve up or down. If it curves down you have a hook. If it curves up it is hogged. both are bad. It should be straight with a nice sharp edge at the transom. If you have this it can be fixed by reshaping the hull.

If that is not the case try moving weight around in the boat. Planing hull boats like to have the center of gravity as far aft as possible. You can move it too far aft, but usually the problem is too much weight in the bow. Things like heavy anchors, chain, and other gear stored forward can cause this. One way to check this is get a couple of large weights, 50 pounds each will do. Put it near the center of the boat. run it. Move the weigh aft 4 or 5 feet. Does it still porpoise? move it farther aft. Try different positions.

The other is the engine set up. How high on the transom is it? Where is the cavitation plate in relation to the bottom of the boat? It should be at or below the bottom. What is the trim? Try triming the engines in and out and see if that helps. Is there any set back? set back is how far the engine is mounted behind the transom. Engine set up is something you can experiment with but it takes time and patience.

Also do you have the right props? And which way do they turn? Inward or out ward? Some times that can make a real differnece.

Last but not least talk to other owners of these boats and see if they have had this problem and how they solved it. BoatUS has a thread on their forums for specific makes of boats. Try asking there.

[keysdisease]

That year and size HydraSport should have trim tabs, a little tab should stop it in a heartbeat.

While not fixing the inherent problem, it will should stop the porpoising

Keysdisease

[sottorf]

i don't agree with IKE's comment that hook or rocker are bad. i have designed hulls for boats doing 65 knots that incorporate rocker and if done corrrectly does not cause problems.

I think what IKE is trying to say is that any local wobbles in the hull are bad. you should examine the hull all over with a straight edge and look for local curvature in the hull.

is the boat porpoising in calm water or in waves? my experience is planing boat instabilities usually show up in flat calm water long before they do in waves. how it starts can sometimes give clues to its origin.

if you find no local curvature in the hull then the best way to fix it is with some trim control device. Porpoising is always associated with high trim. Personally i would just fix an interceptor to the transom that is fixed. It is simple to do wont ever break like trim tabs often do and can be fabricated yourself. you should use the smallest interceptor than eliminates the problem. you can do this by trial and error.

Hope this helps...

[Ike]

This just shows that some things in boat design are open to opinion. That's why we have BoatDesign.net. However, I have a passing familiarity with Hydrasport boats and they are not suppose to have curvature, wobbles or whatever in the aft planing surface. It's supposed to be straight.

Sottof is right though about trim tabs. They would more than likely help. Although I am a bit of a purist and think that the hull should be designed and built so it doesn't need "help", I am also a realist and know that most hulls do need help. Trim tabs are a good solution. But it is always best to sort out any problems first before trying to solve it with an add on.

[sottorf]

I am not familiar with that hull and agree with IKE the best would be to have a hull that works without add-ons.

Actually it boggles my mind how a standard production boat being built in series can have such a problem. This should have been picked up and corrected before the boat went into production.

Or has this particular boat been modified in some way that induced this problem?

[Ike]

Actually these hull problems occur for a number of reasons.

First is popping it out of the mold too soon before the laminate is cured. When the hull cures and sets up out of the mold all kinds of strange things can happen.

Second, and fairly common is the boat is not properly blocked or cradlled when on the trailer or on the hard. (or for that matter, not properly supported on the production line.)

Third, too thin a laminate, that develops hard spots where interior structure contacts the hull, and the hull works when underway.

Another reason for a warped hull is hanging too big a engine on the transom, although I don't think that is the case here, but I have seen it happen in boats in this size range. What it amounts to is the structure is not strong enough to support the weight of the engines, so the stern sags.

[hmattos]

We had a similar problem with one of our Explorer 7.0 metre - 23 foot - RIBs - see www.explorermarine.co.uk - when we fitted it with twin 150 engines and had the console and twin batteries fitted very far back, and a high CofG fixed sunshade.
Our usual layout leaves the console and therefore the helmsman nearer the centre of the boat, but in this special layout the boat porpoised from 20 mph upwards - it should have been a 50 mph boat. Our diagnosis was to put 4 people in the boat and the problem solved as they moved forward.

We could have solved this by fitting trim tabs, but the customer wanted fixed hull planing surface extensions which we fitted to the transom either side of the twin engines.

Each GRP moulding was 250mm long X 340mm wide (10 inches X 13 inches ) and were set at a 1 degree down angle since our tests showed this angle to give a very stable ride across all the speed range.
Most of our boats now have this feature ( but set at 0 degrees) built into a fully moulded transom.

Good luck

Hugh Mattos
Chartered engineer
www.explorermarine.co.uk

[Guest625101138]

Porpoising is not necessarily the result of poor build quality although this could contribute to it.

I have attached a screen dump from a web site that analyses the forces on a planing hull. The output shown identifies the instability associated with porpoising if the CoG is too far aft.

It would be interesting to load the numbers for the Hydrosport. I have made a guess for the 7m RIB with two whopping 150HP motors fitted.

You can do your own analysis on how 4 crew going forward would improve things. Will confirm the theory.

Fitting trim tabs moves the centre of lift further aft so has the same result as shifting weight forward.

Rick W.

[hmattos]

Rick,
Thanks for your efforts.
I would be pleased to put some actual numbers into the plot which you have given, but not being a Savitsky expert, what are the un defined dat variables and what is the output ( square root (c!/2) -1 ?

eps?

Regards
Hugh Mattos

[Guest625101138]

Hugh
If you go to the site:
http://illustrations.marin.ntnu.no/h...ing/index.html
and hold the curser over the green text "figure" near the bottom it will give a diagram that explains the other factors not noted in the nomenclature.

Once you see this diagram it starts to give you an appreciation of how the forces might move about to cause an instability.

I will be interested to see if this helps solve the problem.

Rick W.

[sottorf]

the stability criteria of savitsky is for prismatic hulls assuming no curvature in the hull and also does not take into account the effect of spray rails.

The method is purely based on a critical running trim angle. If the running trim angle exceeds the critical value then the boat is supposed to be unstable. However if the calculated trim is below the critical value, it does not necessarily mean that the boat will be stable but then one should look more at the finer details of the ull shape. THere are some papers written by Donald Blount (just google Donald Blount and Associates) which is on their website in the "press" section. His papers talk a bit about what effect small geometric changes to the hull shape have on stability

[keysdisease]

Hello gentlemen,

I have a couple of comments about trim tabs:

Ike:

Although I am a bit of a purist and think that the hull should be designed and built so it doesn't need "help",

I would agree with this statement if boats operated in a static environment. Boats of the type we are discussing (24ft o/b planing) operate in a dynamic environment with constantly changing conditions. Vessels of this size are sensitive to weight distribution, wind on the beam, and sea conditions, etc. The vessel is designed to run on an even keel within a range of attitude and while that can be maintained in test conditions, it is generally not possible in the real world. While trim tabs are often employed to band-aid flaws, they most definitely have a place on a well designed vessel as well.

Being able to adjust trim while underway will result in the vessel operating at the attitude it was designed to, regardless of loading or conditions. This will result in increased efficiency and a more comfortable ride, something that cannot be achieved in all conditions without asking people to move around or asking the wind to change direction Trim tabs will also allow the vessel to remain on plane at lower speeds, usually resulting in increased range and better fule economy.

sottorf:

It is simple to do wont ever break like trim tabs often do

While it is true that something that is installed has a much better chance of breaking than nothing, modern trim tabs are very reliable if sized and installed correctly.

sottorf:

i would just fix an interceptor to the transom that is fixed.

I am 99% certain that a 2005 24ft Hydra-Sport has trim tabs as standard equipment.

keysdisease

[Ike]

Keys, I certainly can't disagree with you. That was why I made that comment about most boats needing help. Conditions change, weight distribution changes, loads change, all of which can be adjusted for with trim tabs.