Fixed hull extension trim device?

Gary N

Gary N

New Member
Oct 27, 2019
16
Montreal
Boat Info
380 AC
Garmin GPSMAP4212, Raypilot RP650
Mercury Ocean Runner RIB 10'. Tohatsu 20HP
Engines
CAT 3126TA
Here is an unusual extension to a 2000 SeaRay 380AC with CAT 3126TAs. It is about 30 inches long fore-aft and it covers the width of the transom from the outboard extremities of both prop pockets. it is installed with a 9 degree down angle in relation to the keel. There are also fixed trim tabs about 1 to 2 inch wide, molded in at the trailing edge that are about 30 degrees down. It appears to have been professionally installed, and to help getting on-plane. I am suspecting that this has a significant negative impact to the achievable WOT rpm, and on top speed, due to induced drag. There is no manufacturer's ID plate or part number anywhere to be seen. As the two SS struts connect the device to the bolt-on
IMG_4742.JPG
IMG_4743.JPG
swim platform, adding stress to the platform when planing, leads me to believe this is not Sea Ray design. The props are 24 x 27. The Sea Ray spec is 24 x 25. This does not help.

Has anyone seen anything like this? I am thinking of removing it.

Thanks for any input.
 
Interesting. But I wouldn't necessarily count out a hull extension as automatically a bad thing. From your post, it sounds like you haven't run the boat. If I was purchasing the boat, I'd want to get some hours on it before making a decision. There's a good amount of weight there in the back of the boat - it may very well be helpful. Point is - give it a good try for a while before making a decision.

How long has this setup been on the boat? What does the owner say about it? From what I can tell in the pics, it looks like the transom is taking the brunt of the force (which is good) - are there any signs that the platform is pulling away from the transom?
 
Boat is in brokerage, and I have not discussed this appendage with the broker or owner. There are no signs of any mechanical issues, it appears to be properly installed.
You are correct, I have not run the boat. The sea trial showed the boat not making WOT. This could be a good reason, along with the ~300rpm loss from the 2 inches of additional pitch from the props.
 
It could very well be an explanation for the RPM loss. However, it also "appears" that there is some slime growth on the paint - which could also account for that, depending on how much there is.

Just playing a little devil's advocate here... the increased running surfaces (obviously) would have been done to get that heavy back end out of the water better... but maybe they then found that the stock props were too small. If the engines aren't running as well as they should... or there's enough slime... that certainly could explain the RPM drop.

The additions certainly don't appear to be done by someone who just did it all willy-nilly. There appears to be some thought put into it. With that mindset, it makes me think that it is beneficial.

On the other hand, maybe they found out afterwards that it didn't help like they thought it would. But at that point, logically thinking... it certainly would have been easy to cut at last some of it back off. So if it didn't help, why didn't they cut it back off (or at least some of it)? Which, brings us back to the thinking that it must be beneficial. But of course, that's making an assumption that whomever did this was using logic to begin with! :)

I would think you have a very fair question there to ask of your broker - and if the owner is willing, maybe talk to him/her directly.
 
Lazy Daze said:
It could very well be an explanation for the RPM loss. However, it also "appears" that there is some slime growth on the paint - which could also account for that, depending on how much there is.

Just playing a little devil's advocate here... the increased running surfaces (obviously) would have been done to get that heavy back end out of the water better... but maybe they then found that the stock props were too small. If the engines aren't running as well as they should... or there's enough slime... that certainly could explain the RPM drop.

The additions certainly don't appear to be done by someone who just did it all willy-nilly. There appears to be some thought put into it. With that mindset, it makes me think that it is beneficial.

On the other hand, maybe they found out afterwards that it didn't help like they thought it would. But at that point, logically thinking... it certainly would have been easy to cut at last some of it back off. So if it didn't help, why didn't they cut it back off (or at least some of it)? Which, brings us back to the thinking that it must be beneficial. But of course, that's making an assumption that whomever did this was using logic to begin with! :)

I would think you have a very fair question there to ask of your broker - and if the owner is willing, maybe talk to him/her directly.
Click to expand...
 
I like your reasoning that the modification must have been beneficial. I expect that this modification would indeed assist getting the boat on plane, but at the expense of top speed, just like a large trim tab would do the same thing. Maybe the owner considered this a good trade off. However when it comes to selling the boat, well, now it does not make WOT, and so the questions start. As you suggest, I will ask the broker to see if I can obtain any info.
 
Gary N said:
Maybe the owner considered this a good trade off. However when it comes to selling the boat, well, now it does not make WOT, and so the questions start.
Click to expand...
Agree. Quite honestly, this is something that the broker should have foreseen and known the answers ahead of time so you wouldn't be left wondering. The last thing you want is for a potential buyer to be left questioning things.

For the moment, let's go with the assumption (which is fair) that this add-on caused a little to be lost on the top end, yet cruising speed/efficiency was increased. If the broker would have disclosed that right away, most likely this would have been seen a positive selling point... rather than questioning what's going on and whether there are also (possible) engine issues.
 
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I’m not any kind of engineer or anything but I own a 410 Sundancer with Cat 3126’s.
The diesels do make the boat a$$ heavy. Because of this, I’m considering adding drop fins to my tabs this winter to help a bit. After speaking to Bennett last year I may add a second set of tabs over the prop pockets too if the drop fins don’t get me where I want to be.
But after running my boat for 3 season my feeling is that getting that a$$ out of the water would result in an increase in speed. That’s how much it feels like she is dragging that butt through the water.
I can only imagine that what I’m experiencing is amplified in a 380 aft cabin.
Whatever that is on that 380 looks to have been done with some serious thought and engineering.
I’m thinking they changed the pitch from 24 to 27 because after they installed that thing and lifted the butt out of the water they saw an increase in RPM’s.
It sounds like you may have some time and possibly money invested in checking out the boat.
Hopefully the current owner can shed some light on the modifications and loss of RPM. Or at least enough info for you to decide if you want to invest in a survey of the boat and the motors. I don’t think I would rely on what a broker told me about that stuff though. That’s a lot more than a minor modification, it’s pretty unique and a loss of 300 RPM is a lot for those Cats. I’d think that type stuff might be above the knowledge level of most brokers.
 
I have never seen anything like that. Is it made of fiber glass or stainless steel? If it was stainless.....that is at least 400lbs of metal.

If I had to guess maybe they had a jetski on the swim platform at one point? Something caused them to go way beyond adding drop fin tabs or another set of tabs.
 
I seem to recall someone on this forum having something similar added to their boat. It wasn't as beefy looking as this, and maybe not quite as big, but it was a pretty good sized, fixed, piece added right in the center between the props. IIRC, he was quite happy with the performance increase.
 
The entire assembly is fiberglass. Its possible that the underlying structure is SS but I expect it to be plywood, covered in 'glass. There is a dinghy attachment on the extended swim platform, but not large enough for a jet ski. As you say there must have been a compelling reason to engineer something this elaborate. I have asked the broker for info.
 
Gary N said:
Boat is in brokerage, and I have not discussed this appendage with the broker or owner. There are no signs of any mechanical issues, it appears to be properly installed.
You are correct, I have not run the boat. The sea trial showed the boat not making WOT. This could be a good reason, along with the ~300rpm loss from the 2 inches of additional pitch from the props.
Click to expand...
Was this sea trial done by a surveyor for you? What is the correct WOT rpm for these engines? What was the WOT rpm from the sea trial?
 
Here's a photo of the one on Beachcomber. It was done to move the boat's wake further aft. That was needed because the marina put on a swim platform that dragged in the water when getting up on plane and coming off plane. Basically, the SWP was too long (front to back) for the design of the hull.

The extension consisted of a stainless piece bent to match the deadrise of the hull. It is held in place by the two struts you can see in the photo. Outboard of that on each side was a trim tab extension that just about doubled the size of the tabs.
It was an ingenious fix and solved the problem by moving the boats stern wake about 2' aft.
Trim-Tabs-and-Hull-Extension.jpg

Hope this helps.
 
Hi Guys,
Here is an update.

I get 2600 rpm WOT, 200 rpm off the CAT spec of 2800. This is with 24 x 27 inch props, which are 2 inches over the originally installed 24 x 25. The appendage definitely helps the hole shot as there is barely any pitch-up, when transitioning from displacement to planing mode. The downside is that I think the boat's pitch angle is too low (bow down) when on plane. The wake starts at the front of the boat rather than about 1/2 way back. I never need to use the trim tabs to get on plane like all the other boats that I have owned. May be forced to buy a see doo to hang off the back to compensate. :)
 
Gary N said:
Hi Guys,
Here is an update.

I get 2600 rpm WOT, 200 rpm off the CAT spec of 2800. This is with 24 x 27 inch props, which are 2 inches over the originally installed 24 x 25. The appendage definitely helps the hole shot as there is barely any pitch-up, when transitioning from displacement to planing mode. The downside is that I think the boat's pitch angle is too low (bow down) when on plane. The wake starts at the front of the boat rather than about 1/2 way back. I never need to use the trim tabs to get on plane like all the other boats that I have owned. May be forced to buy a see doo to hang off the back to compensate. :)
Click to expand...
Now THAT'S thinking like a true boater!
 
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