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sgdiv7
07-31-2001, 09:47 AM
Can anyone help me with the max length the prop can be from the strut safely. The prop shaft is 1". Boat porpoises with all the extra HP. I have to use alot of peddal to keep the bow down. I can move engine forward 6" max. and I guess i could make trim plates longer if I need to. What do you guys think. looking for suggestions.
Bill

058
07-31-2001, 03:36 PM
I don't know what kind of hull you run or the length of it but this is what works for me. Both my 18' flats [Hondo & Howard] have the prop within 3/16" from the strut. I like to have the strut support the shaft and prop as much as possible rather than having most of the support on the shaft alone. Both boats take a nice set with no porposing at all with the plate in any position. Perhaps you need to extend and/or re-adjust the cav. plate.

126driver
07-31-2001, 07:26 PM
I agree with 058. Check the plates, run it again, then move the prop shaft back one nickel at a time if you're still not happy. Somewhere on this site there used to be a good article on setting the cav plates for a baseline. I think Wilkes wrote it. Haven't looked for a long time to see if it is still here or not. How far away is the prop from the strut right now? We started with the prop almost touching the strut and have it moved back 6 nickels right now. Maybe more to come. IMO play around with everything else before you start moving the motor. Small adjustments can make a big difference. Change one thing at a time and run it. If you still don't have any luck, get the tape measure out and post the length from the transom to the back of the motor plate and the back of the v-drive plate as well as the length of the cav plates so we know what you're dealing with.
Good luck.
Scott

sgdiv7
08-01-2001, 05:40 AM
Thanks for your help.
The boat is home made 17' 6" runner bottom. I set the plates level with the bottom of the boat. the changes I made to the boat when I put the new motor in was I put in a powerglide trans, and I went from a 15 gear to a 29 gear. The old motor was stock 454 and the new motor is a 454 w/alu. heads and 871 supercharger, so I figured I added 100 LBS. to the back of the boat. I'll get the other measurements this week and I'll post them.I really don't want to move the motor forward any more because I can already feel the heat from the headers Its hot enough when its 100+ deg outside, lol. I do want the boat to be right

Sangerboy
08-19-2001, 12:44 PM
I agree with the previous posts, 3/16" is the ideal distance from the back of the strut to the front hub of the prop. This allows enough room for the prop to move forward 1/8" inch or so under hard acceleration due to driveline flex. If your running 500 or more hp I wouldn't put more than 1/2" of space, to much chance of whipping which could break your shaft with major consequences at high rpm.

ponponracing
08-22-2001, 09:55 AM
Just keep in mind that the further you are from the strut, more chances you are going to break the prop shaft. And this is costly.

Bad Ass Flat
08-22-2001, 11:05 AM
The only way you are going to stop the porpoising is to either put more power to it to carry the nose ( you can drop gear to achieve that) or put steady pressure on the down peddle at WOT. It needs something tohelp carry the nose or something to keep it from porpoising (down peddle pressue)

Sangerboy
08-22-2001, 08:45 PM
You guys are pretty sharp. I had to much bowlift with my old bubbledeck and had to move the prop back quite a bit to get it to set at neutral plate. I didn't like the thougt of shaft breakage at 90+ so I'm having a new strut made to match the new prop position. Besides the safety issue if you've forked over decent dough for a good prop you don't want to lose it.

superdave013
08-22-2001, 08:51 PM
Sangerboy, Who is making your strut? Barber Welding makes some nice ones. They made a rudder for my drag boat and it was bitchin.
If you need to, contact Doug Barber @ 562-928-2570.

Sangerboy
08-23-2001, 07:40 PM
superdave, Jim Lange at Hi-Tech Marine in Simi Valley is building my strut for me. He did some bottom work for me, and we moved the v-drive forward and went from a 10 degree strut to a 6 1/2 degree strut. I still had to move the prop back quite a ways to get the boat to set, and it was too far back to just weld an extension to the existing strut so we opted to build a new one for safety. I'll contact Barber Welding for a new rudder, I cut mine down to 10" but it is still chrome plated and I wouldn't mind having it be made of a stouter material. Thanks for the info.

superdave013
08-23-2001, 07:46 PM
Sounds like you have the right guy helping you. When I was just getting into racing I ran against some of the boats he setup. Lew Larson is another very, very good guy at setting up flatbottoms.

Sangerboy
08-26-2001, 06:55 PM
Superdave, do you have an address or phone# for Lew Larsons shop? I don't think you can ever have to many resources. I'd like to file that information.

Pro Gas Flat
08-27-2001, 10:51 AM
There are so many reasons why a flat will porpos, its mind boggeling. The most common reason is lack of power, it will never go away if you dont make enough beans to carry the the nose, no matter where the prop is. Second is strut, engine, and v-drive location all have to be correct for the amount of power the engine makes. Some boats are set up for normally aspirated engines and others for blown, not only for locations, but strut angles as well. The weight of the boat will also make a huge difference, as well as how much gear you are trying to pull. Biggest mistake that everyone makes in a river boat is too much gear. The motor will never burn the propellor enough to let the boat wheelie into a nice set on the ential hit and from there on out the rest of the pass the boat will porpus forever. Spring pressure also is very critical, the more spring the plates have the better. Also the propellor is a huge factor, no one prop is good for everyone's flat, everyone's set ups are different, you just have to break out the wallet and start your very own prop collection, about $700 a pop. You just have to try alot of different things, but I can tell you one thing your not a nickel or two changes away from solving the porposing problem, way more info on the set up is needed as well as engine information. Thought I would give my 2cents, Ive seen more people go the wrong way than the right, with their set ups.
[This message has been edited by Pro Gas Flat (edited August 27, 2001).]

Bad Ass Flat
08-28-2001, 12:11 PM
Pro gas flat what makes you an authority on boat porpoising? Are you an expert? You no doubt drive an all white boat. Like you own a lot of propellers

Leon
08-28-2001, 01:11 PM
Hey Pro Gas Flat. Hi you must be slowboats friend from dragboatracing.com. He talks about you all the time and says you guys are very interested in his jet hydro project. I think you shoyuld give him some advice. He needs all the help he can get trying to go 100 so he doesn't lose the family fortune.

Pro Gas Flat
08-29-2001, 11:21 AM
Bad Ass Flat, the only way I can show some "Hang Fin" to the crowd is when she's on her 10th hop or so, then she's really flying. You know the same way your ride looks out of the gate, setting into a nice doink, doink, doink, let it get in your face alittle quit being a bitch and drive it like you have a pair.

Bad Ass Flat
08-29-2001, 01:09 PM
Pro Gas Flat, I'm giving her all she has. Whippin her hard. It is good to see you on. I can't stop stop doinking it. How about the river this weekend?

PGF545
09-13-2001, 02:02 PM
Doink, Doink, Doink... thought that is how you drive a PGF. Hang Fin for a little while then doink it to the right, then doink it to the left and then flop it right on the bottom....thats right isn't it???