Bryan Rose

I am posting this to keep all informed with this experiment, lots of people always asking what if I add a shorty.....well here is some info and I will keep you all posted through the dialing in.....more to follow next week

Standard Leg 8 inches deep
Shorty leg 6 inches deep
Boat 27 ZX Donzi / 500 EFI twin step
Smooth to 6-12 inch chop water


Well I finally got the boat wet after the switch and it seems the resuts are perplexing. I di not have a new prop to test , so I used the one I have now a 24 inch Tired Bravo.


Before with the standard leg I was able to spin it to 5400 revs and attain 68 MPH.

2 inch shorty I was able to spin to 5600 or higher , I was on the rev limiter so i do not know if there was more left in it or not. I think ther is because it would pull to 5600 easily.
Speed was 69 MPH....Bad slip #

Time to go up in pitch.


the best the boat has seen before the shorty was with a 25 inch M+ to 71 =72 MPH at 5000 -5100. good slip #

So as it seems I have increased the slip # but only Marginally increased the speed. Boat handled well , with no blow out or problems getting on plane. I have lost some of the bow lift as expected so I am now working with a prop person to get it dialed in. We are heading in the direction of 26 box stock Bravo I to get a base line then if it is right fine tune a labbed 28 to get the desired results....at this point it is still an experiment, but hey it takes time and work to get a boat dialed in.

Is it a success ? or is it a failure...? at this point the jury is still out on this perhaps a drive spacer is in order, need to find the sweet spot.....who knows, any one have 1 inch spacer they want to let me try for the sake of this experiment?

What has been your experience and what are your thoughts on this whole deal? I welcome any advice and look forward to hearing from You




Thanks
Bryan

klaw

Bryan what was the starting x-dim

Griff

Where are you taking your measurments from for the 6" and 8" deep?????

They need to be taken from the propshaft center to the bottom of the hull to have any relevance.

Also, how are spinning a 500EFI at 5400 and 5600rpms????? The rev limiter is at about 5300rpms.

Bryan Rose

[COLOR=DarkRed]I have modified the ecm and raised the rev limiter to 5600?????I am taking it from the lowest part on the keel down to the tip of the bullet on the gearcase thought this was what I was doing....8 inches to the propshaft center on a standard leg and 6 inches to the propshaft center with the shorty, mind you this is from the bottom of the keel down to the prop shaft center.

Bryan Rose

Not sure of the actual X dimention that term gets thrown around a lot. I have only measured the distance from the keel to the prop shaft...
Standard leg 8 inches
Shorty leg 6 inches

Bryan

Vinny P

I always understood the "X" dimension to be the distance from the crankshaft centerline to the bottom of the hull. The measurement you are changing by using a shorty drive is actually the propshaft height.

Griff

That is DEEP!!!!!!!

Most high perf v bottom boats will run propshaft height in the 4-6" range.

Bryan Rose

Thus the shorty................I am trying to get it right ......need mo bigger prop.........

Big Block Billy

I have been through this dilema before. Unfortunately, it's going to require some experimentation with props, and maybe spacers. I would try to bring several props with me and change them in the water, instead of hauling, and then maybe try the shims if not happy with any of the props.
My friend's boat had a Konrad conversion, and we went through alot of grief. It turns out that the Konrad was 2" shorter than the TRS. We could not get the 38' Scarab to plane off good , it had excessive cavitation. We tried switching rotation of the drives and props inward and outward, several times with different props, got two inch spacers, same crazy testing and swapping around, and then went with 1" spacers.
It turns out that the boat that my friend had had a high X dimension from the factory.
Konrad makes the 540 drive 2" shorter than trs, and may be fine for a Cig. or other hull that was not setup with a higher X dimension. The drive being shorter was intentionally done. You can always add shims, but you cant shorten a drive.
You may try a 5 blade herring prop, you need to grab more water. Cavitation is wasted foward motion. it's like spinning your wheels, Although you have a bravo , the same principals apply... BBB

Bryan Rose

Like you say it takes time and effort to get these things dialed in. I agree with that statement in full. I am going to keep going forward with this project and look forward to doing it, still early and I have about 2 months to get it dialed in and ready for the season. I would prefer to have a properly set up boat to gain speed rather than just throw massive amounts of power to it. Alittle more difficult to set it up but I think in the end really worth it. I will be testing a full barage of props and spacers I am sure before saying " Yes I have got it " I have a feeling the sweet spot is going to be 1/2 to 1 inch lower than where I am now ....onlt time will tell.

Keep the thoughts coming guys I appreciate it.

Thanks
Bryan