HPDE Sponsons for a 23' GRP runabout

Hi,
We have a 23' Deep V runabout (24° transom) that is unnervingly unstable at rest.
The boat was built too light.
Can HPDE be used as bolt on sponsons?

 

Welcome to the Forum Rotto.

Do you have any photos that you can post of your boat please?
I presume that she is one of a series or class of boats that were built - do you know the name, and how many other boats were built to this design?
What is her beam (width)? I am guessing a bit under 8'?
Do you know of any other sisterships that are also 'unnervingly unstable at rest'.
How unnerving is this? Does she heel dramatically if you move from the centreline to the side of the boat? If so, does she then 'stiffen up' once she has heeled over a bit?

 

Hi Bajan, It's Voyager Marquis, built 1980 in Perth Western Australia. They are all much the same. The stringers and ribs are a hollow moulded grp grid. These originally were powered by a stern drive; either 485 MerCruiser 4cyl, 165 6 cylinder or occassionally 228 V8. It has an aluminium pod and a 2 stroke 200 Mercury outboard. I leave the bung out and let the pod flood or the problem is worse. Beam is 8'4". The problem is the weight, or significant lack of it. It heels over to where the chine is immersed and then stiffens up, but that's a bit of heeling. I figured a sponson down the chine right aft on each side about 3"-4" square & 4' long would help, and wondered if HPDE would be suitable.
Adding 800-1,000lbs would help too but that would require a structural refit and more hp and it's just not worth it.

 

The inboard Mercruiser engine along with the outdrive leg would probably be a lot heavier than your current outboard engine.
You mention leaving the bung on the pod out, to allow it to flood - but once it has flooded, do you put the bung back in again? I am thinking that if you leave the bung out, then the water will probably drain out once you are up on the plane?
I am no expert on deep vee planing boats, but there are gentlemen on this forum who have a lot of experience with them - I am sure that they will be along soon and offer constructive suggestions.

You noted that 'It heels over to where the chine is immersed and then stiffens up' - does she do this when you are underway in a straight line, or just when you want to alter course?
Maybe your boat is 'chine walking'? here is a good description of chine walking -
Scream And Fly Magazine - A Walk On The Wild Side: Chine Walking - Why It Happens and How To Fix It https://www.screamandfly.com/content.php?150-A-Walk-On-The-Wild-Side-Chine...

 

No it's fine underway if a little bouncy from being too light, at rest it's tippy. It doesn't go anywhere near fast enough to chinewalk. It just behaves like a cork at rest, thusly sponsons to increase WL beam a bit.

 

I have experience with flooding/draining hulls.
They can really help reduce tender response characteristics.
Remember, tender is not a poor quality in most cases.

Sorry, I don't have anything further to add.

 

Rotto, do you have any 'local' forums in Australia relating to fast deep vee planing boats where you could perhaps seek opinions from owners of sisterships?
It would be interesting to see what owners of boats with inboard engines say, in comparison with your experience with your outboard engine.

 

Are there many around?
Does everybody have the same problem you do?
What have others tried to relieve the "problem"?

 

If you primarily use it for fishing, maybe a wet well midships could help. Not a big one, or you'll get structural issues, and pumped - no hull holes. Or a mid/forward extra fuel tank or water tank. Giant well stocked esky.

 

Changing to an outboard from an inboard really raised the CofG, and decreased the weight. Hence the change in behavior. Restore the weight to where the original designer intended, and it will behave better.