Reinforcing or modifying forward chine flats for slamming

[mitch184]

I've got a 26' fiberglass offshore boat. It is a 22° hull but it has 5" wide chine flats that run almost all the way to the bow. As such, in certain tight chop, the waves hammer the chine flats and boat pounds and slaps HARD unless you make adjustments to your direction and trim. The construction of the hull is 3/8" solid glass/1" core/1/4 glass.

This is a long shot, and I couldn't think of a way, but is there anything that could be done to lessen the impact and noise? I have the boat torn apart so I have easy access and was just curious, but changing the hull form or anything on the exterior of the hull is not something I want to do. Thoughts that crossed my mind were: adding another section of 1/2" core with a thin laminate, hitting it with spray foam on the inside or adding weight to the chine flats in the area.

Again... long shot, but figured i'd ask if maybe someone has done something creative. thanks

 

[jehardiman]

The hull is 22 degrees in the forefoot? And you're running in the Salish Sea? What speed does it slam? And a picture or boat manufacturer would be helpful.

FWIW, almost all planing boats will slam in specific "tight chop" spacing and speed...the nature of the beast.

 

[mitch184]

The hull is 22 degrees in the forefoot? And you're running in the Salish Sea? What speed does it slam? And a picture or boat manufacturer would be helpful.

FWIW, almost all planing boats will slam in specific "tight chop" spacing and speed...the nature of the beast.

22° at transom. Deadrise isn't listed for the bow but I measure it out at around 40. Will snag a picture of the boat inside and outside this afternoon. Its an Osprey. 2'-3' chop pounds hard.

 

[jehardiman]

22° at transom. Deadrise isn't listed for the bow but I measure it out at around 40. Will snag a picture of the boat inside and outside this afternoon. Its an Osprey. 2'-3' chop pounds hard.

Which length? And do you know your running trim angle?

https://www.ospreypilothouse.com/wp-content/uploads/2016/02/CRW_9506.jpg
https://www.ospreypilothouse.com/wp-content/uploads/2016/02/CRW_9532.jpg

 

[mitch184]

26'. Not sure on exact running trim angle but typically they run with a lot of trim as you can see in those pictures. The keel has a slight camber to it which I don't know hurts or helps. The wide chine flats so far forward create a lot of lift in the bow it seems

 

[Ad Hoc]

I.... It is a 22° hull but it has 5" wide chine flats ...

5" or 127mm, is a tad on the wide side, but not excessive.
All planing boast in certain conditions experience chine slapping (as also noted by JEH)...the running attitude v wave amplitude/period always plays a part too.

 

[mitch184]

Only real way to help it seems is trim tabs and motor trim to keep the bow down.

I guess ultimately maybe I'm thinking is there a way to strengthen the area where the boat is slamming? Or is it even needed. I just have everything torn apart and it would be easy to add some glass, foam or core.

 

[mitch184]

Here you can see the wide chine flats FWD... those are the noise makers..

 

[baeckmo]

In my view, this is a really bad design. Looking from the side you can see that the chines operate at a very high angle of attack at all speeds; instead of deflecting spray to the sides and downwards, they meet the inclined wave surface "flat on". This creates a full slamming pressure wave over a large chine area, resulting in high levels of vertical acceleration.

You may reduce the slamming somewhat by adding a blending radius between bottom and chine, large enough to "spare" about 50 mm of the outer horizontal chine. This will reduce the "slamming area", yet retrieving the spray deflecting quality. Note that the edge between chine and vertical side must be kept sharp.

 

[DogCavalry]

Definitely a sub optimal design. I just bought a gallon kit of fairing compound, so my first thought was to build up that area from outside. Adding a wedge shaped buildup to ease that flat towards 22° like the deadrise at the transom. No doubt if I'd just bought a hammer I'd be thinking "nails!".

@baeckmo 's suggested remedy is better. Take more fairing compound, but would reduce slamming more effectively. At that point on my boat the non-tripping chine is about 70°. No slamming, but a tendency to whisker spray.

 

[mitch184]

Baeckmo, you are correct. The boat has many good attributes but this is its major downfall. Basically making a large radius/fillet between the bottom and chine flat and leaving 50mm of "flat" at the outboard edge of the chine.
If a some what simple construction of structural putty and a few plies of glass re coated with gel coat would be strong enough, it doesn't seem like it would be terribly difficult to modify.

Dog,
Do you mean in the bow the chine flat would be completely covered so there is no "flat" but rather it is 70 degrees above horizontal?

In both your experience, would you take the time to do this? This mini project on the boat excites me more than the other glass work I've been doing for sure.

 

[baeckmo]

Well, I'm an "alu-builder" and if it were a metal hull, I would reshape the chines, no hesitation. But you have to consult the "glass-guys" about how to attack the radiusing and weigh possible improvement against investment in time and material.

 

[mitch184]

Understood. If I could get away with fairing using structural putty I could have it done in a weekend. Likely I would need to add fiberglass, which should be done with a vacuum bag since it's exterior, to make it strong enough so that may be a next year project.

 

[DogCavalry]

@fallguy or @Rumars come to mind. I would follow @baeckmo 's idea. Reduce the width by careful filling. You can make epoxy fairing compound that is really really strong, with the appropriate fillers.

 

[mitch184]

Basically something like this... which starts at the bow and then goes aft potentially to where the chine becomes horizontal or roughly where the resting waterline is? Will likely be around 1.5" thick in the center.