X dimension ponderings...
 

Looking for info on x dimension set up. I'm re rigging a Formula 357 with bravos (check out the link in fiberglass and paint) and want to know how to do all the math from start to finish. I would like to get half way between conservative and aggresive. Formula said only that the x dimension was 15 1/2". I have the width figured out and I have old x dimesion from the trs rigging. I figured that the crank to propshaft measurement difference between the bravo and trs is 2 1/2" shorter. So If I wanted to keep it consevative I could go off those numbers by simply lowering the old trs x dimesion by 2 1/2". How much is to much...I know that I can always get spacers laterif I get them to high, but would rather get real close if possible.

Thanks Joe

http://www.offshoreonly.com/forums/s...d.php?t=145051

Re: X dimension ponderings...
 

The X dimension is the distance for the crank center to the bottom of the hull. With a X of 15-1/2", the drive would be quite deep by today's standards.
I believe a Bravo is like 22-1/2" from crank center to prop center. That would have the prop shaft some 7" below the bottom. Personally, I don't think you need to be lower than that.
The TRS drives were mounted deep back in the day. Prop technology was also vary different.
I'd keep the engines at the same height that they were with the TRS drives.

Re: X dimension ponderings...
 

MOST NEW BOATS and my old like it between 5 1/2'' to 5''

Re: X dimension ponderings...
 

hey joe i did the same thing you are about to do i have a 34 scarab so basicly the same hull or similiar,i searched for the same info for month and i found it in in a article in powerboat bob teague said on older boats like ours to set the propshaft depth at 4 to 5 inches below the hull i set mine at 4 1/2 and it worked perfect for my boat you will have to get 4 blades like i did, no cavatation, great getting on plan even with 8 people

Re: X dimension ponderings...
 

Thankyou! Man this is a subject that is very hard to decribe with out a drawing so I made one. My only problem is I can only go so high with my engines. (blowers) They are 1/4" awayfrom the hatch right now and I don't want to cut holes in it. So with this setup my propshaft will end up 5 3/4" deep. That is a little deeper than I wanted but you can't have everything in life...The prop should still surface a little. I can always go with shorties later when I hit the lottery! Still might consider raising the extra inch and raising the hatch a bit. That would give me 4 3/4". Which I would be happier with. It's hard to believe that Formula engineers came up with a propshaft hight of 8 1/4". They knew about surfacing props back then...the boat came with them!

Joe

Re: X dimension ponderings...
 

Prop technology has come a long way since your boat was build. The day of the Merc. Clever is long gone. The new four and five blade prop allow for better plaining and low speed use, where the old clever use usless with raised "X" dimensions. I agree with everyone try to get as close to that 4.5 inch number as possible.

Re: X dimension ponderings...
 

So that three blade cleaver was not meant to be a surfacing prop? I had always thought that was the reason for the pointy tips... I use hyromitive 4 blades, I know they could not have been surfacing at the depth they were at.

Joe

Re: X dimension ponderings...
 

I rerigged my 1984 Excalibur 31 from TRS with the merc trans to bravos and couldn't be happier.
My boat has a pad bottom and a notched transom. My stock X dimension was 18-1/2 and I upped it to 19. The bow used to bury itself at speed before and scrub off speed. Now it's a little trickeyer to get on plane (with my 26 pitch 4 blade Bravo1 props) but other than that I love it. It carries the nose beautifully.
I thinke my prop centerline to bottom of the boat was like 5-1/2 (I never measured it before disassembly), but now it's 2-1/2.
Good luck,
Dan

Re: X dimension ponderings...
 

Thank god there is someone out there who has done this. Thanks for the post. I'm affaraid to go that high without the notch but I did do some more measuring today and I think I can get another inch out of the new set-up I forgot that as you go back (as I will be by chucking the trs trasmision) I will get at least one more inch of hatch clearence. 4 3/4" will be better than I thought I would be able to achive when I started measuring.

Joe

Re: X dimension ponderings...
 

Is a prop shaft centerline measurement of 4.5 inches below the keel the optimal measurement or what you can expect from the trs to bravo swap? My 271 fastec is at 4.5 & I wonder if a 2" shorty would be of benefit? Would that be a wise step in the 3 year plan to more HP?

Re: X dimension ponderings...
 

hey joe that should be good that should be great like i said i did alot of research before i started cutting holes and it was not easy at all to get someone to give you a dimension but,on straight bottem boat this seems to be a good number,because if you go to high you will not get enough prop bite to lift the bow,stepped bottom boats create there own lift so on those boat the x dimension is alot higher,and dont need as much positive trim to get the nose up

Re: X dimension ponderings...
 

Hey if Bob Teague says between 4 & 5" then thats what it gonna be!

Re: X dimension ponderings...
 

It depends on the boat, not all boats automatically do better with a higher X dimension. The higher you go, the more leverage you lose to lift the bow. The boat stays wetter and my not do as well.

Re: X dimension ponderings...
 

put in in the way it is, and see how it runs, and then if you need to shorten it go with a shorty lower unit. My old 311 with 502's had B1's and runs like a dream. insptech now owns it and has had trouble free fun with it.

Re: X dimension ponderings...
 

Spacers are alot cheaper than the shorties, I think that 4 3/4" is not to crazy, but if it is, the spacers will get me deep enough. I like tinkering anyway...Do you know what the prop shaft hight was with the shorties on that 311? How did it handle?

Thanks Joe

Re: X dimension ponderings...
 

Here’s an odd thing I forgot to mention.
I have the offshore engine mounts. The way things were with the merc trans, the bottom of the motor plate rested right on top of one of the smaller strengthening stingers running under the engine.
With the new set-up, I moved the engine centerline up ½ inch so I figured there would be space between the plate and the stringer……………nope.
After spending 2 hours aligning each engine, both engine mounts ended up right back down on the stringer (4 inches rearward from where it was before).
The angle of the engine in my particular boat was different. Placing the front of the engine lower than the rear (when compared to how it was with the TRS and merc trans).

Everything was aligned properly, the bravo alignment tool slid in and out with ease. The drives slid right in without a problem, and then right back out after 18.6 hours of trouble free use.

I’d definitely move your crank centerline up.
Good luck!
DS

Re: X dimension ponderings...
 

Well Zanie...you are correct! I did some more measuring last night and found exactly that! The further back you go the deeper the engine gets. But I have another issue. The further back you go the shallower the upper deck hatch opening gets too. My offshore mount will no longer fit at the bottom either. So I have to go the Bravo mount. I see that Mike (offthefront) has them in his. I have worked on lots of HP boats with them...I like them for their ease of adjustment and I can get my engines on the work stand in 45 minutes. Go over and check out the rotton transom thread. I am going to redesign that whole front bulk head to be a more strucural part of the boat wich in turn will tie the stringers together better than they were before. This way I will feel better about loosing the offshore mounts.

Joe