moving the motor forward

[tahitijet]

when i did my setback i used the driveline to determine how far back i set the motor Since a BBO came out of it years before the BBC went in and I installed a rail kit at the same time it was the only way i could really determine placement. Well the boat seems to handle pretty decent so far.
My concern now is i'm wanting to install a jet a way. Does anyone know what the common effect is of moving the motor forward that far to allow room? I'm already running the shortest driveline I think i can. So the motor will have to come forward and i'm assuming it will sit slightly further forward then the BBO did prior to the set back
can I expect any handling changes? any insight would be appreciated
thanks

[steelcomp]

It may come out of the hole a little quicker and get on a plane sooner, but since the weight is further forward, it will have more difficulty carrying the nose, which means more boat in the water, which means a little slower top end. If you're looking for speed, you'll have to go back through and re work your set up...more nozzle angle, back cut shoe, a little up on the ride plate, etc. If you don't have any of this hardware, you'll be stuck with what you have.

[cyclone]

While i agree with Steelcomp that adjustments to the underwater hardware (rideplate/shoe) will be needed after moving the motor forward....I dont agree with the statement that you'll slowing your boat down. If anything, moving the motor forward should help you in that the boat wont have the tendancy to drag the ass as much. Given enough power to carry the bow, the boat can run quicker without having as much of the rear dragging in the water. Most boats can carry the nose pretty easily, the truly quick and fast ones also can carry the rest of the boat as well. the goal should be a flat attitude with just the intake in the water, not a nose up, dragging the transom through the water attitude.
my 2 pennies in the bucket.

[steelcomp]

While i agree with Steelcomp that adjustments to the underwater hardware (rideplate/shoe) will be needed after moving the motor forward....I dont agree with the statement that you'll slowing your boat down. If anything, moving the motor forward should help you in that the boat wont have the tendancy to drag the ass as much. Given enough power to carry the bow, the boat can run quicker without having as much of the rear dragging in the water. Most boats can carry the nose pretty easily, the truly quick and fast ones also can carry the rest of the boat as well. the goal should be a flat attitude with just the intake in the water, not a nose up, dragging the transom through the water attitude.
my 2 pennies in the bucket.
I'll see your two, and raise ya two....I totally agree, but with the qualifying statement "given enough power". This is why I don't like using a lot of nozzle angle to "carry the nose", since you're also driving the ass end down, causing more drag. If you have the power to carry the nose, then by moving the motor forward, you do lighten up the ass end, but you're still moving the weight forward, making it more difficult to carry the nose. If you're already having trouble getting the boat out of the water, it won't help to move the motor forward. I've had this discussion with many people, and there are definately two schools of thought. However...Given the Tahiti hull, unknown HP/ hardware, and the question was "theoretical", that's what my comment was based on. If his bottom is stock, he's probably already fighting a hook, and moving the motor forward will only compound this. Like I said, IMO, just to move the motor forward, without changing anything else will probably slow the boat down a little on top end. The point may be moot, since it sounds like he has no choice. It'll be interesting to see what it's effect is.
Tahitijet...do some documentation and record the changes as you go along. This could prove to be some real useful info for everyone.
Cyclone, now there's four cents in the bucket, 'caus I saw your two, and raised ya two. :wink: :coffeycup

[Squirtin Thunder]

caust I saw your two,
What is that ??

[cyclone]

I'll see your two, and raise ya two....I totally agree, but with the qualifying statement "given enough power". This is why I don't like using a lot of nozzle angle to "carry the nose", since you're also driving the ass end down, causing more drag. If you have the power to carry the nose, then by moving the motor forward, you do lighten up the ass end, but you're still moving the weight forward, making it more difficult to carry the nose. If you're already having trouble getting the boat out of the water, it won't help to move the motor forward. I've had this discussion with many people, and there are definately two schools of thought. However...Given the Tahiti hull, unknown HP/ hardware, and the question was "theoretical", that's what my comment was based on. If his bottom is stock, he's probably already fighting a hook, and moving the motor forward will only compound this. Like I said, IMO, just to move the motor forward, without changing anything else will probably slow the boat down a little on top end. The point may be moot, since it sounds like he has no choice. It'll be interesting to see what it's effect is.
Tahitijet...do some documentation and record the changes as you go along. This could prove to be some real useful info for everyone.
Cyclone, now there's four cents in the bucket, 'caus I saw your two, and raised ya two. :wink: :coffeycup
good point. guess we need to know more about the boat then. better to know than assume.

[Ken F]

My thoughts would be that first we need to know a little about the driveline he's running and go from there. assuming the driveshaft is in fact as short as possible, He'd only have to move the engine forward about 4" I believe to make room for the jet-a-way, which probably wouldn't make all THAT much difference in a Tahiti would it?
Ken F

[UBFJ #454]

The Optimum Location of a motor in a boat is solely a function of weights & balances ... Both static and dynamic. All hulls are Different even if they are the same kind, same manufacturer (different mass distributions within the layup, etc.). Best place to start is to determine the static CG of the boat fully rigged w/out the motor ... Determine the CG of the motor, factor in the motor's Hp (shifts the static CG ... Dynamic Cg), bottom (lift characteristics) and the leverage characteristics of the bottom hardware (ride plat angle, shoe type & depth, etc.) ... All In All it's a 'Black Art' consisting of A Lot of Trial & Error and, Some Times ... Hair Pulling ... Each Boat Is Different and the Faster you want to go (like Racing) the more Nit Picking It Gets.
Sorry No 'Real Answer' to your Question other than Weights & Balances ... Both Static & Dynamic.

[Jake W2]

I think if you have an H bar you can take out the center part or the H and not need to move the motor at all.Dont know if I would do that but I think that is what Duane said.
Jake

[Fogged Gullwing]

Good topic
I moved My pump out 5"'s and still had to move it a 1/2" forward and to get the gap from the snap ring and yoke to allow for hull flex. Does your boat really need it? Is it going over 90 mph? Most people that use one are going fast with a hull that will throw you out of the boat at speed if the motor dies.
I really like the neutral advantage that it gives but if I'm not running the number I would not run one. I'm going to pop off my boat also. I really don't feel like a 95 mph ride on top of water that feels like concrete!!! I've bounced at 65 and it knocked the wind out of me.
Good Luck
BK

[Mr.&Mrs.Budlight]

Over the years, I have blown several motors, and locked them up at 6,000 rpm plus. I have never had my boat do anything stupid. My shoe is 1/8 th above the keel line in my CP gullwing! What are you guys doing that makes it so dangerous? Or am I just too safe? My boat was rock safe to 100mph. Budlight

[Jake W2]

Budlight are you running a back cut shoe or a stright shoe?
Jake

[Fogged Gullwing]

Hell Bud,
I don't know the last time I had my boat out it was 80 top's and if it died it skipped along without fuss. But now with a lot more power and shoe ride plate and a tight pump I 've been told to watch this boat from more than one person. My fond memories allowed me to go to sleep at 80 so I guess I'll get my wakeup call when I run it. Hope all is well with the new boat!!!
Later BK

[tahitijet]

Here is a little more background.
Hull- The bottom has been blueprinted, pump set back 3", shoe, rideplate, and diverter. I have a droop i will be installing as soon as i can get on the water and get a base gps number on the current setup.
drivelin- is an h-bar. I cannot shorten it anymore. The motor is as low as possible in the boat and because of the deep oil pan there is a small differance in the pump shaft hieght and crank height. So i need both u-joints to couple them together.
Motor is a 540 chevy dynoed at 700hp.
currently the boat plans quickly very little bow rise and rides flat. water breaks right at the front of the motor.
2 reasons for wanting the jet-a way. 1 the nuetral feature and 2. I do expect to see high 80s to low 90's with this boat. And even if i don't touch these speeds I still like the safety feature it provides. I'm sure even a boat that shuts down perfectly straight at 90 mph could be apt to make a hard turn with the right water conditions.
My main concern was moving the motor causeing a porpoise or bow steer or some other ill effect that i couldn't tune out. Sounds like motor placement has more to do with which end of the boat is harder to lift.
I think i have enough power to handle that issue with proper tunning of the boat hardware. Hopefully the weather will cooperate and stop raining and in the next few weeks i can break out the gps and do some testing.
this is good reading if you have any other insight or info please post.

[Fogged Gullwing]

Good Reasoning!!
Just wanted to know why you wanted it. It sounds like your prepared to take on the challenges yourself. Good Luck!!!!! I'm basicly doing the same trying to get my stuff right. Alot of work with little reward! I've allready spent 200.00 on tools I don't need!! Buy something and install it 50% increase!!!
Good luck with yours BK