Riva classic speedboat - problem planing (porpoising)

[daiquiri]

First of all, I want to tell you that I really like your boat. Take good care of that beauty.

The plate which now is missing has nothing to do with hydrodynamic behavior of your boat. It's only purpose was to protect the hull area above the prop from abrasion.

I have contacted the nearby Riva RAM (Riva-owned restoration and maintainance company for their boats) facility in Sarnico and they have confirmed me that boats from that series have historically been equipped with max. 220 HP. As far as they know, you are the first person who has installed 330 HP on that boat, so I guess it's a good occassion to open a bottle of Champaigne now. Cheers!

A somewhat bad news is that they don't consider it safe to put more than 250-270 HP in that boat, as it was not designed for that engine power.

So, the area inside of which you can search the solution to your problems is essentially bounded by 3 elements (as mentioned by the others too): a flat-V hull, a too powerful engine, a wrong prop.

Without diminishing the value of contributions from other people (me included) who have given you some valid indications about what measures and modifications you can undertake to alleviate the problem, I believe that PAR has given you a good general advice. Find a person experienced in boat tuning (Greece is a big and maritime country) and bring him on board. He will probably have the possibility to evaluate the problem better than a group of guys sitting in front of a PC screen on the other side of the world. Keep us informed!

Cheers

[keysdisease]

Beautiful Boat.

With all due respect to Alik the installation of trim tabs or interceptors will end your problem instantly with added benefits. As both of these devices have no effect whatsoever on the hull until they are deployed it would be impossible for them to make your problem worse.

Regardless of the hydrodynamic stability reasons for this behavior either lifting device will interupt the cycle by forcing the transom up and the bow down and then keeping it there.

Steve

[dckelly]

Originally Posted by daiquiri

First of all, I want to tell you that I really like your boat.

Thanks a lot

I'm going to try to find someone, as you say, but I think I'll probably resort to some for of trim tabs. If I do, I prefer to use the interceptors, because they seem to be the least intrusive. Since she is based on a boat and lifted with davits, two big plates sticking out of the back will be difficult to handle.

Do you have any experience with these interceptors - do they work as well as tabs?

[PAR]

Now that I see you bottom, there are two things I don't like. First is a slight amount of rocker in the a aft portion of the keel. This should be dead straight and from what I remember of this hull, possibly with a slight hook. The other thing is what seems to be the forward end of the hook that has been either reshaped with bottom fairing or the boat is slightly distorted in this area.

Have the folks at Riva look over those photos and see if they see anything.

[keysdisease]

The Q/L intrerceptors from Volvo should be readily available in your area and I agree less intrusive than trim tabs. Interceptors performance will be very similar to trim tabs except at slower speeds where tabs have the lift performance advantage.

Porpoising is something that we encounter here in South Florida quite alot. Due to the year round boating and numbers of boats in the area this will often occur when there is a repower with additional power or overpower which is very common. I find it more common on outboard powered boats and it can occasionally be "fixed" with the O/B's trim function or with propeller modifications like additional cup.

This statement is from the Bennett trim tabs website:

http://www.bennetttrimtabs.com/guidetotrimtabs5.php

Correcting Porpoising
Porpoising is a condition more common in faster, performance boats. As speed increases, the bow repeatedly rises out of the water until gravity overcomes lift and the bow bounces down. Press "Bow Down" in half second bursts. As the trim tabs deflect, the porpoising subsides and your speed should remain the same or increase. Only a slight amount of trim tab deflection should be necessary.

I don't take everything I read on a manufacturers website as gospel, but this matches my own experience and that of others in helping people with porpoising.

Steve

Originally Posted by dckelly

Thanks a lot

I'm going to try to find someone, as you say, but I think I'll probably resort to some for of trim tabs. If I do, I prefer to use the interceptors, because they seem to be the least intrusive. Since she is based on a boat and lifted with davits, two big plates sticking out of the back will be difficult to handle.

Do you have any experience with these interceptors - do they work as well as tabs?

[Alik]

Originally Posted by keysdisease

With all due respect to Alik the installation of trim tabs or interceptors will end your problem instantly with added benefits. As both of these devices have no effect whatsoever on the hull until they are deployed it would be impossible for them to make your problem worse.
Steve

I agree, there is a possibility that tabs or interceptors could help, due to change of oscillation frequency. For eliminating of proposing any factor that changes frequency can help.

But considering that they have tried to shift CG already and there was no result, it might not be sufficient.

Anyway adding wooden strakes on bottom is cheaper option compared with Volvo's tabs