Upgrade 400/410

For the guys that are cruising between 24-25 mph burning 22gph... have any of you tried to re-prop to see if there’s any speed to be gained.

I’m asking b/c the 99-02 340’s were shipped from SR with 3 blades and there’s significant mph gains (about 3-5mph) when going to 4 blades, and since I increased my tabs, I can get up on plain at lower RPM too.

suss1173 said:

For the guys that are cruising between 24-25 mph burning 22gph... have any of you tried to re-prop to see if there’s any speed to be gained.

I’m asking b/c the 99-02 340’s were shipped from SR with 3 blades and there’s significant mph gains (about 3-5mph) when going to 4 blades, and since I increased my tabs, I can get up on plain at lower RPM too.

Click to expand...

The speed you run and the pitch of the props with diesels is dictated by the engine loading (combination of the weight of the boat, gear ratio, condition of the bottom/running gear etc). Heavier boat/dirtier bottom will result in slower running speeds at a given rpm. Overloading or over propping a marine diesel is a good way to shorten it’s life or kill it outright. So should not be done on diesel powered boat.

If your interested, Google “sbmar.com engine load” and a couple good interesting articles will come up.

There was a very interesting post on The Sharrow prop design that might change this math, but I don’t think they make props in the right size. Even so, a set would run $20k!

suss1173 said:

For the guys that are cruising between 24-25 mph burning 22gph... have any of you tried to re-prop to see if there’s any speed to be gained.

I’m asking b/c the 99-02 340’s were shipped from SR with 3 blades and there’s significant mph gains (about 3-5mph) when going to 4 blades, and since I increased my tabs, I can get up on plain at lower RPM too.

Click to expand...

dtfeld said:

The speed you run and the pitch of the props with diesels is dictated by the engine loading (combination of the weight of the boat, gear ratio, condition of the bottom/running gear etc). Heavier boat/dirtier bottom will result in slower running speeds at a a given rpm. Overloading or over propping a marine diesel is a good way to shorten it’s life or kill it outright. So should not be done on diesel powered boat.

If your interested, Google “sbmar.com engine load” and a couple good interesting articles will come up.

There was a very interesting post on The Sharrow prop design that might change this math, but I don’t think they make props in the right size. Even so, a set would run $20k!

Click to expand...

Totally agree with dtfeld on this one.

However, at the risk of hi-jacking the OP's thread I'll comment on my real world experience with the 4 blade props we purchased and installed on our 340DA. We had significant gains with regards to handling around the docks, low speed maneuvering, and getting out of the hole but definitely lost a little on the top end. Probably around 3-5 MPH. It's a trade off.

-Tom

The diesel 410’s of my boat’s era came with 4 blade props.
Oddly enough, Acme props are highly recommended by my local prop guy and I know a couple of people who have had great results with them, but when I called Acme last winter about a recommendation for my boat they recommended their 3 blade props for my particular boat.
Sort of threw me for a loop and scared me off of them.
I guess the morale of the story is that there is no single answer that covers all boats.

suss1173 said:

For the guys that are cruising between 24-25 mph burning 22gph... have any of you tried to re-prop to see if there’s any speed to be gained.

I’m asking b/c the 99-02 340’s were shipped from SR with 3 blades and there’s significant mph gains (about 3-5mph) when going to 4 blades, and since I increased my tabs, I can get up on plain at lower RPM too.

Click to expand...

How did you increase your tabs? Did you have room for wider tabs?
I’d love to increase mine but there is really no room to replace my current tabs with wider tabs.

JVM225 said:

How did you increase your tabs? Did you have room for wider tabs?
I’d love to increase mine but there is really no room to replace my current tabs with wider tabs.

Click to expand...

I had a guy weld new tabs onto the bottom of the original tab. IIRC, I increased the overall surface by 35%.
It was a very easy mod. I didn’t gain any speed, but it dose allow me to stay on plain at lower speeds/rpm’s.

The pictures below were not the final version. I eventually redid them so the gap between the transom and the tab wasn’t as big as what’s in the pictures below. If you need anymore info feel free to PM me and can talk on the phone.

suss1173 said:

I had a guy weld new tabs onto the bottom of the original tab. IIRC, I increased the overall surface by 35%.
It was a very easy mod. I didn’t gain any speed, but it dose allow me to stay on plain at lower speeds/rpm’s.

The pictures below were not the final version. I eventually redid them so the gap between the transom and the tab wasn’t as big as what’s in the pictures below. If you need anymore info feel free to PM me and can talk on the phone.

Click to expand...

Thanks for the pictures. Unfortunately, doing something like that isn’t a great option for me because my tabs turn down for an inch or so on the outboard sides and up an equal amount on the inboard sides.
I’d have to cut away the drops on the outboard sides to weld something on and if it had an adverse effect I would be screwed because the factory tabs are a parallelogram shape instead of the more common rectangles.
I’ve called Bennett about it and they said that the Sea Ray engineers special ordered that size tab and shape but they don’t know why.
They suggested either of two things:
First was trying to install 3” drop fins on my existing tabs, but cautioned that I should try to speak to someone with the exact same model before doing so to see what their results are because of the shape of my tabs and the curved transom. If there was an adverse effect I could remove the fins and fill the holes with the screws I used to retain them.
Second, they said I could consider installing a second set of tabs that are 12” x 12” over the prop pockets. The problem with that is that I would have to figure out the optimal spot and angle for them.
I’m not any kind of engineer but I feel strongly that if I could get the diesel heavy stern up a bit there would be some pretty good performance gains at cruising RPM in both speed and fuel efficiency.
Like most people who run these boats, I initially felt that getting the bow down more would be the answer. After running the boat for a while and playing with the tab positions I realized that the bow goes down plenty, but the stern stays squatted down too much regardless of tab position.
My swim platform is always much lower than the water surface at cruising speed and always gets soaked.
It seems like the Sea Ray engineers went with an identical tab setup for both gas powered boats and the much a$$ heavier diesel boats. Again, I’m no engineer but I think that if you have two identical hulls but you put a much heavier load in the back of one of them you would need to compensate with different tabs.
While the diesel boats “go pretty good” with the factory setup, I think they may have left some performance on the table in the interest of using the same tab setup on both gas and diesel boats.

JVM- I also think you would see some gains by putting tabs over the prop pockets too. The longer you can make the boat, the better the gains will be, so the longer the tab, the better I think it will be.

Do you boat on the LIS or the south Shore?

suss1173 said:

JVM- I also think you would see some gains by putting tabs over the prop pockets too. The longer you can make the boat, the better the gains will be, so the longer the tab, the better I think it will be.

Do you boat on the LIS or the south Shore?

Click to expand...

South Shore. My dock property is in Lindenhurst.