General sportfish hull-tunnel design questions

[buckshot]

I have a 2007 Luhrs 41 Sportfish Convertible.
To start things off, I do not need the whole spill on the THT forum about these boats. I am aware and have read it.
I have not had any of the crazy issues noted in that thread. The boat performs reasonably well for 40k lbs at 41ft long. Slightly top heavy and a bit of a handfull in a steep following sea but I have been on worse.
The boat has a cruise of 22-25 knots at .47 to .5 mpg or 44 to 50 gallons total burn per hr. She is powered by twin 720hp diesels.
I do have a general question about an area in the hull ahead of the tunnels. It is where the shafts exit the hull, there is a deep hard edged pocket that I feel is introducing a lot of cavitation that would flow directly into the prop tunnels. This would be robbing me of lift and sending dirty water to the props. The boat also has a good bit of cavitation in the prop wash while trolling, I am going to try and dip a GoPro while trolling to get some footage of where most of this turbulence is comin from.I have thought about glassing in these areas and put in a short shaft tube with a cutlass bearing. This is a design I see on a lot of boats in the yard.
My question is, do you think there will be any noticeable benefits? Here are a couple of pics and my quick rendering of the "fixed" area.
I have also noticed most of the sportfish boats with tunnels have a short tunnel extension glassed in, what are the predictable changes a tunnel extension would provide? I have included a pic of one of these extensions, this is NOT my boat- just for reference. This would be a relatively cheap and quick hull mod.
Thanks

 

[Fiberglass 1 Inc]

I have a 2007 Luhrs 41 Sportfish Convertible.
To start things off, I do not need the whole spill on the THT forum about these boats. I am aware and have read it.
I have not had any of the crazy issues noted in that thread. The boat performs reasonably well for 40k lbs at 41ft long. Slightly top heavy and a bit of a handfull in a steep following sea but I have been on worse.
The boat has a cruise of 22-25 knots at .47 to .5 mpg or 44 to 50 gallons total burn per hr. She is powered by twin 720hp diesels.
I do have a general question about an area in the hull ahead of the tunnels. It is where the shafts exit the hull, there is a deep hard edged pocket that I feel is introducing a lot of cavitation that would flow directly into the prop tunnels. This would be robbing me of lift and sending dirty water to the props. The boat also has a good bit of cavitation in the prop wash while trolling, I am going to try and dip a GoPro while trolling to get some footage of where most of this turbulence is comin from.I have thought about glassing in these areas and put in a short shaft tube with a cutlass bearing. This is a design I see on a lot of boats in the yard.
My question is, do you think there will be any noticeable benefits? Here are a couple of pics and my quick rendering of the "fixed" area.
I have also noticed most of the sportfish boats with tunnels have a short tunnel extension glassed in, what are the predictable changes a tunnel extension would provide? I have included a pic of one of these extensions, this is NOT my boat- just for reference. This would be a relatively cheap and quick hull mod.
Thanks

Ventilation and cavitation are two different things. What you describe is ventilation and yes, it does happen. The shaft alley, at planing speeds, is a low pressure area so any air running along the bottom (and there is plenty of that) gets pulled into the alley making it a very aerated (or ventilated) place. Once you're up and going, say 16 to 20 knots, the air will start to track down the shaft and into the props as you mentioned. I've discussed this with many NA's and they say don't worry about it because the aeration only occurs on the inside 10% of the "disc" (their term for the diameter of the propellor. Most of the actual thrust is toward the outside diameter of the prop and therefore the loss near the hub is negligible. But if you want to cover the shaft alley you don't need to change anything as long as you have shaft seals (internal water supply to the bearings which I think all the Luhrs 41's had). You can make a template out of sign board, cut a slot and hole for the shaft, hot glue the temporary panel in place over the alley,put mold release on the surface and glass it in place. Then pull the whole thing off, separate the form and the real piece and that will give you a fiberglass cover that can be screwed in place and removed for bearing work.
The tunnel extensions are a much more interesting discussion for another time.
,,

 

[buckshot]

Thank you for the response,
I thought about making a temporary set of covers just as you described. I am hoping to find time this off season when we pull for bottom maintenance.
I am interested in the extensions, I know I see them quite often around the yards so they must do something. I also know it was some kind of a "fix" Luhrs did to the earlier 41's to address some issue.

 

[baeckmo]

What is the prop dia and rpm? Asking this because it is common with these vessels to have a rather high load on the propellers (thrust related to disc area and dynamic pressure). With a high prop loading you will have an increased risk for ventilating air entering the tunnel from behind, along the tunnel roof. This is effectively stopped by an extension of the tunnel, but sometimes at the cost of reduced rudder efficiency. Instead, a small interceptor edge along the tunnel exit can be a very good medicine. Dimensions depending on tunnel size and prop loading.

The side edges (tunnel to bottom) should have a generous radius, because the inflow to the prop disc at high loading to a high extent comes from the sides. The shape I see in your case is trigging flow detachment and increased turbulence. Of course this also goes for the shaft pocket and the "unmotivated" edges found there, but those are not the main culprits.

 

[buckshot]

I apologize that I do not remember the exact distance from the prop tip to the tunnels. I seem to remember around 4 inches or so.
The props are 27x34.5 P -4 blades from Wildcat. I cruise at 1800 to 1900 rpm and the tranny's are ZF's at a 1.75 ratio. I had 5 blades on the boat and she was a pig. The new 4 blades gave me 5-6 knots on top and a solid 4 knot improvement at cruise.
However, with 1440 hp on a 40k lb boat I feel like there may be another knot or so to be had without major mods. This off season I plan on stripping the bottom so this would also be the perfect time to make any of these fiber glassing mods. Here are a few more shots showing the props/hull.
In your comments regarding the extensions. Are you saying the props could pull air in from the back of the boat and that is what the extensions are mostly for? This could cause prop slip and poor rudder response? I have definitely felt like the boat could use larger rudders, I have even thought about making them slightly larger when I change rudder shaft seals. So maybe the tunnel extensions would help instead?
If there are no negative results from 3 to 4 inches of tunnel extensions this would be a relatively easy mod to glass in.
So maybe fill the shaft alley and add small tunnel extensions?

Again I really appreciate all the comments

 

[buckshot]

Sorry, just read it again. I see that you said the tunnel extension could CAUSE poor rudder response. So if I considered doing the tunnel extensions then I should also add some area to the rudders? I was thinking adding a couple inches to the trailing edge of the rudders would be correct.
Lee

 

[buckshot]

this is a pic I found of prop extensions on one of the earlier 41's. This was supposed to help solve some problems on the tunnel design in the 04-05 maybe.

 

[kapnD]

Both of the mods you mentioned would likely help, as well as rounding out the tunnel edges. The forward end of the tunnel has a nasty step in it too.
Id want to closely examine propeller modifications after that, 4” of clearance is extremely generous, possibly creating space for turbulence to occur..

 

[buckshot]

Thank you all again for the responses, I am learning a good bit here.
Lee

 

[Fiberglass 1 Inc]

Thank you all again for the responses, I am learning a good bit here.
Lee

I've never seen a 4" blade tip clearance in a tunnel before. We've worked on a couple of the 41's in the past and don't remember seeing that clearance before. As kapnD said, that is quite generous. Also, keep in mind that any large squared off trailing edge will create a low pressure area and any ventilation usually finds its way into that low. I know it seems counter intuitive but I have found tank test videos of little vortices traveling from the transom INTO a 30 knot stream of water flow and attaching to the rudders, causing total ventilation and loss of control (100 foot Navy mine sweeper, Carderock Institute). Your rudders have that large square trailing edge and are very likely pulling air in from the transom.

 

[buckshot]

ok, so adding 2-3 inches to the rudders and streamline the trailing edge should be put on my list?
To make sure I understand about the rudders, at cruise speed 20 to 25 knots the rudders are probably pulling air from the transom. This would make them less efficient and they would provide less control.
I am not sure about prop tip clearance I will try to check them sometime to make sure I am sharing correct information.

 

[Fiberglass 1 Inc]

Not sure if tapering the trailing edges would be necessary if taking other measures first (like the tunnel extension). You might check to see if the prop diameter is what the boat came with, maybe that was changed for some reason.

 

[Cacciatore]

Didn't you have these problems with the old propeller?They started as soon as you changed the propeller correct?

 

[buckshot]

I am not sure I am having "problems", I am just looking for small improvements that could be made. The boat is running great but there is probably room for improvement. Yes, the boat ran horribly with the props that were on it when I purchased the boat. The props Wildcat tuned for me made a huge overall speed/economy difference. The wildcat props are slightly larger than the original 5 blades also. Maybe an inch if I remember correctly. With some of the comments it seems I may be able to increase the prop diameter even more. From my basic understanding you basically want to run the largest prop you can.

 

[kapnD]

Read Dave Gerrs “Propeller Handbook” to get a good overview of propeller engineering.
It’s not an engineering textbook, even I was able to easily understand it!
Many factors contribute to correct propeller selection, and final decisions usually require trial and modification.