BenPerfected

I agree that a lower X might correct the handling issue, but it will likely lower the top speed opportunity. Which way you go depends on your budget and goals. I don't see 3.5" below the pad as a real high X. For sure, trying a 1" spacer is a low cost option.
Some manufactures try use the exact same bottom design yet vary the power that can yield a 75 or 95 MPH result. The bottom becomes really important to optimizing top speed past 90 MPH.

502stang

ok..thanks guys...The boat ran 65 new. It had a stock 502 mag and a stock bravo one that had the prop shaft a little over 8 inches below the pad. It had no bad traits at that time. I now have a 850 HP 840 FT/# of torque engine in the boat and an extension box and the prop shaft is 3 1/2 inches below the pad. This is NOT a step boat. IT only has the pad. The boat runs to 75 to 80 with no bad traits what so ever! It porpoises really bad at 80 and at 90 it feels like the stern is going to come out of the water when the bow goes down.

What kind of hull work are we talking?

The boat has plenty of power left so I am going to try the spacer but I did not think that 3 1/2 inches below the pad was too low either.

I am also going to try a Maximus prop and see how that works. There are a couple of guys that have almost the same hull but they both have 2 steps and they have more trouble with chine walk and feeling like they are floating around on marbles!! They have gone to 380 tabs to help settle them down but that is not going to help me keep my bow down so I am going to have to go a different way. Thanks again for the help!!

Griff

What brand/length boat???????

What prop are you running now?????? more blades=more stern lift and less bow lift.

A spacer will give more leverage to carry to the bow. 1" can make a big difference.

What is your propslip%??? If the slip% is high, lowering the drive with a spacer can actually increase your speed.

502stang

It is a Donzi 25 ZX which is really 24.5 including the swim deck. The prop that has gone the fastest is a Hydromotive H-6 that is lab finished and is a 29p. The boat ran 90.2 at 5400 rpms. I think that calculates to 9% slip. So that prop hooks up very well. The second best prop was a Turbo 5 blade OXP 30p box stock. I was not nearly as good but that is before I upped the power more. The Hydromotive runs out perfect to the 75 -80 mph range. It does not carry the bow but it just rides on the pad up to that speed with no problems. After that when the speed increases the boat will not skim along on the pad and start to leap more than porpoise. Then if you try to trim up at all it starts to porpoise really bad. This is why I have only been able to get the boat to 90.2 mph. It gets to doing it so bad that it seems like something bad is going to happen if you push it any further. The 3 and 4 blade props do not like this setup at all and just blow out.

598Boat

I had a similar problem on a 28' cigarette. I made a set of stainless trim tabs slightly larger then the 380's It stopped the problem all together. I lost a lot of speed. It stuck the stern harder into the water. It wasn't freeing up. I only lower the trim tabs when taking off and then raise them all the way up. I only have to lower then if running at low speeds. Otherwise they are always up. I don't remember, but think my prop was around 8" below. Wasn't thrilled about the speed lose, but have to make it safe first.

BenPerfected

502,
Modifying the bottom (likely lengthening the inter strakes assuming yours don't run the full length of the boat) is not difficult and shouldn't have a total cost of any more that a new set tabs or rocker plates. My boat had the exact same issue as you are having with porpoising as we increased speed. The bow would lift and then fall off because the inter stakes were not long enough (at speed) to give the needed surface/support to hold the bow...hence porpoising. In other words, we were just running mostly on a the flat surface of the boat. We don't have a pad, but I seems to me that you might need more strake contact with the water to balance/hold the boat on the pad.
The fix (our diagnosis and rough-in work was done at Lake X) was to cut some oak strips the same dimensions as the strake and then screw them in place with a gelcoat paste a get a smooth surface (all work done on the trailer). We then tested and adjusted the length of the strake extension until the boat was stable. This process changed the entire handling of the boat. We started at 83 MPH and we finally stopped testing at 107 MPH (owner was running out of balls). The boat handles better today at 90 MPH+ than it did at 70-75 MPH.
The only real cost was with a FG shop to make the new strake additions when you are finished testing. The actual materials to test with were less than $200. On my boat, the final inter strake was extended 5 ft!!! and ended about 12" behind the CG.
If you decide to not pursue, just fill the screw holes with a gel coat paste (HD or West Marine) and sand smooth...no blood.
Reggie is famous for spend a lot of R&D time on the bottom to improve speed and handling. With the speeds you are running with a 23' boat, a bolt-on fix isn't likely going to give a safe fix...assuming any 23' boat is safe at 100 MPH+

CB-BLR

More blades = more stern lift.

Generally - more blades = less speed if outdrive is set at proper depth.

If you can not hook up a 4 blade... this is a strong indication that your outdrive is too high for your setup application.

Hull drag is far greater than outdrive drag.

Get a good 4 blade, and start lowering the outdrive untill it hooks up well in the midrange and top end.

My boat picked up over 5 mph by lowering my outdrive 1.5 inches.

I did this in .5 inch increments and then tested. When I got to 2 inches of spacer, the boat slowed down... hence the 1.5 inch spacer.

I would start with a 1 inch spacer.

Chris

BenPerfected

502,
Back to your original question...I tried the Arneson Rocker Plates and they scrubbed about 3 MPH mounted even with the bottom of the boat; we could add no speed in any rocker position. They may help some boats but I much prefer the long trim tabs to hold/control the bow when needed. As the rocker plates are mounted flat with the bottom, they add to the wetted surface and on our boat (dead flat bottom) didn't add any measurable benefits....sold them to another OSO'er to try.
We also back to back tested a set of 5 blade Herrings vs. the B-1's , this was just prior to the Maximus being released. At the same X (PS 3/4" below the bottom), the Herring's added +2 MPH vs. the B-1's but the added stem lift made the boat feel unconnected and squirrely. For me, at 100+ MPH feeling connected was more important that +2 MPH.

502stang

Ben..both of my lifting strakes (inner and outer) run all the way to the transom. I have been told by IMCO that is one of the reasons that the stern will not set down and the bow can not stay up. They say the back is digging in and causing the stern to lift when the bow goes up. I think that I am going to start spacing the drive down and see if I can make it work. If I loose some speed that will be ok with me if the boat runs stable and takes a good set at speed.

502stang

Chris...My main objective with this project is to have a smooth acceleration and to cruise at the highest rate of speed at 3500 -4000. This 6 blade Hydromotive is really great at that. It has very low slip numbers across the board. I think that I do not want to go any lower than 5 blades. I am going to try and find a maximus and then start spacing down and see what happens. I think there is a lot of difference between a 4 and 5 blade on cruise speed and smoothness.
Thanks!