Swim Grid Dragging

S

StellaKaruna

New member
Joined
Oct 15, 2020
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4
Fluid Motion Model
R-21 EC
Vessel Name
Stella Karuna
I have a Cutwater 28. My swim grid drags when on plane. Does anyone else have this issue?
 
Bumping delayed post.
 
This is one of my pet peeves with this boat. I think it should have been an integrated full flotation hull extension providing lift not drag. The boat digs a hole because of the fixed drive shaft angle and then the bottom edge of the swim deck drags along the bottom of the hole. Bigger trim tabs might help, I added fins to the stock tabs and saw a slight improvement but it doesn't eliminate the problem. The boat simply won't trim bow down enough to get the bottom of the swim deck out of the water.
 
Hi P. Leeson.

Thanks for the reply. I noticed that you are in Hariot Bay. We were just spent a couple of days in Rebecca Spit. Weather was hot and it was like anchoring in the Caribbean. Lovely area.

Can you tell me what you mean by a "full floatation hull extension?".

I am having to replace my swim platform as it has become cracked and water logged after a few too many rough seas. The swim platform really gets hammered by some of the waves on the west coast of Vancouver Island. Last crossing of Georgia Strait, I bent one one of the dingy standoffs. Yikes!!

I am thinking of replacing the swim platform with an aluminum grid. However, the "full floatation hull extension" idea intrigues me.

I have added 4" extensions to my trim tabs. It helped a bit, but not enough. I am also considering changing the pitch on my propeller to help get the bow down.

Any thoughts on this would help. I think the swim platform is creating quite a drag when on plane and reducing my speed and fuel economy.
 
Pretty normal. I find if I’m not properly up on the plane it’s worse. 3050 rpm higher and she gets her butt out of the water.

Also if you have gear and a heavy dinghy on the stern it makes it worse.
 
This may be a design issue with all inboard Tugs and Cuts. The platform is user friendly and nice to have but there are flaws. I find on flat water it is only an issue when running just above hull speed to digging the big hole wake. The prop wash hits the lip formed to anchor the small fenders. The water floods the top of the platform and causes drag. I installed 18 X 12 planes which help to push the bow down but do nothing to resolve the platform issue. A few observations that I have made in regards to the integrity of the platform. There is wood used for support. Some of the wood is sealed with a thin layer of resin and mat some just with gelcoat. Most of the fasteners were lacking sealant and caused some water penetration. In rough water it tends to have a great deal of upward force applied to it. It is thru bolted into the transom but limited sealant was used so this is an area of concern. I recently looked at a platform that was removed from a C28 (2016) because it was starting to rot and was water logged. The wood used as backing to hold the platform to the transom was soft and the platform was loose.When all the mounting hardware was removed many of the fasteners had no or very little evidence of sealant which caused the platform to rot and water log. I recommend inspecting the platform and resealing all the hardware. The C28 owner replaced his old platform with a new platform. When doing this he applied proper sealant and then sealed everything with epoxy paint.

As far as changing the prop. You are not going to get any stern lift from doing that. I would recommend changing the prop though. Most C28 owners that I have talked to are only getting 3300 to 3350 rpm WOT normal load. You really want that D4 to turn 3550 RPM normal load. To gain stern lift the drop fins will give you more lift that tab extensions. The Drop fins cup the water and add lift. I replaced the 12X12 planes with 18X12 and achieved similar results to the original drop fins that I installed to the 12x12. I would give the drop fins an edge over the larger planes. I also added a spacer block between the trim cylinder and the plane giving me about 5/8" more depth with the tab down. This does help when cruising in rough water heading into the sea's. Do not use this much tab with following sea's.
 
Brian:

Thank you for the great information. I am pricing out an aluminium swim grid rather than replacing it with the same one. I am going to have it grilled rather than being solid so that the water can flow through it when it is dragging. Also am hoping to eliminate the lip which seems to cause the flooding as you mentioned.

Also I didn't know about Drop Fins. Thank you. It sounds like an easy and cheap addition to get a bit of stern lift.

I Marty C. recommended that I read some of your posts on propellers. Great information. I am a bit of a novice at this propeller thing. I now understand about wanting to have the engine running at 3,500 RPM at WOT. I am back home so can't take my boat out until the New Year to see what it does at WOT.

In one of your posts you mentioned the you cupped your propeller. That is something new to me. Could you explain?

Thanks Bill
 
StellaKaruna":l3pvv26g said:
In one of your posts you mentioned the you cupped your propeller. That is something new to me. Could you explain?
Click to expand...

The props have a cup close to the trailing edge of the prop blade. This helps " grip the water" it adds efficiency to the pitch. I had a 17 X 17 with .105 cup. I have loaded my boat down so my engine rpm was marginal 3850 to 3960 depending on fuel and water. I like to see above 4000. I didn't want to drop to a 17 X 16 so I removed some cup .030 this allows the prop to slip and engine turn more revs. I lost approximately 1kt in speed doing this so not perfect but increased WOT rpm to 4040 to 4130 fuel and water makes the variable. The better move would have been to reduce pitch and keep cup. MartyC replaced factory prop with 2" less pitch and he maintained prop efficiency and I believe see's equal to or better performance and increased RPM 3550+ WOT.
 
Many boats now use a pod or hull extension to extend the hull and provide a place to mount the outboa d engine without sacrificing cockpit space. The good ones are fully sealed and actually provide additional flotation as the motor tries to drive the stern down. It wouldn't take an engineering genius to do the same thing for our swim decks! I second the recommendation to add a plastic block between the hydraulic ram and the trim tab so that when retracted the tabs are a flat extension of the hull. This will give you more useful travel to he,p trim the bow down, and with the addition of fins to the tabs will improve things noticeably even if the swim platform drag is a design flaw that just keeps on giving. If Fluid Motion insists on continuing to use this design they should at least make the threshold into the cockpit from the swim deck higher, so that they could also raise the swim deck. A smart person would also "thicken" the aft end of the swim deck on the upper side rather than create a lower lip to scoop water in all but perfect calm hull speed conditions. Sometimes I really wonder about the engineers among us? Too many frat parties maybe.
 
the cutwater 32 has exactly this. its the same basic hull but with a huge sealed swim step. as a result the boat is a lot faster ...that and the 600Hp hanging off the back
 
Could a pod be retrofitted under the swim grid and how would this change the handling? Any thoughts?

I did a quick calculation and a pod across the length of the beam,1 foot deep and 2 feet wide would add about 1,000 LB buoyancy. To get an idea of costs, I noticed you can buy pods which add 200 LB buoyancy for about $400. It could be attached to the swim grid rather than to the transom.

I have dingy, 6 hp motor and propane bottles on the grid. That may weight 200 LB. Then there is the weight of the swim grid and railings. So maybe 500 lbs buoyancy to offset all the stuff hanging off the transom.

However, first I am going to replace my waterlogged, rotten fibreglass swim grid with a grated aluminum one, add the fin tabs and do the trim tab extension suggested by Brian

I am pretty reluctant to mess with the handling of the boat to be honest, but fun to explore. I do love my 2016 Cutwater, but as Paul has said, the bow attitude on plane and swim grid dragging are design flaws.
 
i think for it to be successful it would need proper engineering into the boats hull and not using the existing bonding system.

To be honest Im not sure I would call it a design flaw. Just a characteristic of this boat. These are light displacement powerboats and are susceptible to weight placement. Adding a person or running with a full fuel/water tank vs not changes performance let alone having 200lbs hanging off the back - it will always cause problems.
 
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