Shorter drives?
 

I asked a question in another thread and it sort of got buried.

I know that shorter drives make you go faster, most of the time. They have less drag than a standered length drive. But, does that simply mean you will have less slip and run the same rpm, only faster speed? Or, will it raise your rpm and possibly create a need for more pitch on the prop?

Re: Shorter drives?
 

I'm curious also? I actually pulled the trigger on two IMCO shorties, so I hope it pays off with some better speeds using the same props.

I'd like to know if shorter drives reduce slip allowing more speed while using same props?

Re: Shorter drives?
 

Because they run closer to the surface, I would guess they increase slip. I know a few guys that had trouble planing off after installing shorties. The solution was to install 5 blade props. The shape of the case may reduce slip. I installed nosecones on my old boat with TRS drives. The speed stayed the same but the rpm's fell by 200. I think it was because the nosecone smoothed the flow of water to the prop.

Re: Shorter drives?
 

I really don't think it will increase slip running closer to surface, at least that's not what I'm shooting for. Cleaner water to props, better trim angle and less drag is my understanding of how to gain speed. I do run 5 blade props, so hopefully this will combat the planing issue.

Re: Shorter drives?
 

you won't be gaining cleaner water with shorties unless they are on a box....

speed usually comes with a prop change...

Re: Shorter drives?
 

The speed increase comes from less drag from the gear case. Everything is a trade off. Ususally with elevated drive heights, the boat is a little harder to get on plane. You will most likely have to go to a 5 blade prop of some type, Maximus, Hydromotive, Herring. Water pressure can be a problem getting clean water, not distrubed to the water pickups. You may also loose some bow lift. If your hull doesnt need alot of positive trim it shouldnt matter. It takes alot of testing to get everything right.

Re: Shorter drives?
 

Do you think I'd gain some rpm's on top end with same props though? I can only get about 4800-4900 rpm with my 496 ho's and Hydro p5x's.

I talked to Eliminator and was informed the 28 TD has never really benefitted that much by putting boxes on. They did recommend shorties though.

Re: Shorter drives?
 

you might gain the 100 or so rpm's to get to the 5k you need..

Re: Shorter drives?
 

I'm running Hydro p5x's, so at least I have that going for me... :D

I have a cat so the drives will have low water pick ups on them. Will this still pose as a problem?

Re: Shorter drives?
 

The new prop design technology promotes running surfaced (higher) with actually LESS slip and more overall efficiency. JULIE... WHERE ARE YOU?? You can explain it much better. --- Jer

Re: Shorter drives?
 

She hangs out down in the prop section quite a bit, but I agree, I'd like to learn all I can since I have two shorties on the way.

It will all come down to trial and error which only amounts to more money... :(

Re: Shorter drives?
 

On my boat when I added the shortie the RPM increased 200 RPM and the slip stayed the same. This netted about 3 MPH compared to the standard length drive. I then re-propped and gained another 2 MPH at close to my original RPM with the standard length drive. I would expect to see an RPM increase unless you completely lose your bow lift leverage and the boat is plowing, then you may lose RPM!

Craig

Re: Shorter drives?
 

Thanks Craig. and everyone else too.

I'm running a non stepped V with a transome notch and staggered engines. My prop shafts now when level are about 3" below the V part of the hull (not the notched out part) that is directly in front of them.
That seems pretty deep to me, is it?

Re: Shorter drives?
 

Are you running a cat? I hope I don't end up plowing the nose due to no bow lift. Then it's time to go back to the Bravo 4 blades, then I probably won't be able to plane out... :(

Re: Shorter drives?
 

That doesnt seem too far off Mark. Mine is about 6 inches below and my boat has a notch and boxes. Glad you found a couple of shorties Harper. I found one, so if you know anyone with another -2 Imco tell them about me.

Re: Shorter drives?
 

Payton,
I run a non-stepped V with twins and my prop shafts (when parallel) with the bottom are .75" below the bottom. I get on plane a little slow with Bravo 1's in a 30' hull @ 8,000 lbs. (dry).
I started at 4.25" below (including a 3" spacer) w/27P Mirage props at Lake X. Base line was 97 MPH w/ Imco 2" shorties. Switched to Bravo 1's (same effective pitch; 27p Mirage to 28p Bravo 1). At this point we started reducing the spacers...2", to 1" and then to no spacer. The only significant change was the speed increased each time we raised the drives 1". We ended at 103MPH+. After a re-rig, the following year I tested at .75" below the bottom vs. the final 1.25" from the previous year. No change in speed. In my straight V, 1" below is close to optimum.
Sprague

Re: Shorter drives?
 

Yes my boat is a cat and it gets bow lift with no trim at speed. If you need bow lift it is possible the sorties will not provide enough leverage. Ya never really know until you try! Good luck!

Craig

Re: Shorter drives?
 

Do you want to sell the one?

Re: Shorter drives?
 

Tell me a little more about your setup? Boat, engine, drives, props, boxes ... etc ...

What have you experimented with as far as drives and props?

Re: Shorter drives?
 

It really depends on the boat, what X dim is has on it, and so on. I have a 2000 fountain fever that came with a very low X dim from the factory. When I went from standard length sportmaster drives to -2 shorties I picked up 5 mph and the water pressure went thru the roof. I was turning bravo one 30's around 5100 rmp and after putting on the shorties I was turning 32 bravo ones at 5300. The imco's have a larger water opening so I am going to have to install relief valves to get the water pressue back within limits.

Re: Shorter drives?
 

My boat is a 23' WPM cat with a NA solid roller 598 home built and not dynoed. With a standard length Bravo it ran best with a 30 Bravo at 6100 RPM through 1.36 gears. My x is pretty high and I really didn't think the shorty would help. Luckily I have a buddy that let me try his before I bought mine. With the shorty I turn the 30 6300 RPM and can swing a 32 6000. The slip is essentially the same in all scenarios. Don't you have a 28 Daytona? I have a friend with a 28 and he runs 2" above the sponson on a box. Without a box you will need to be about even with the bottom of the sponson. My propshaft is even to 1" above the sponsons. Obviously mine is a single. Good luck testing!

Craig

Re: Shorter drives?
 

Yes, I have a 28 TD Daytona with 496 ho stockers. When parallel with bottom, my propshafts sit about 1/2 to 1" above the very bottom tip of the sponsons. I have all ready ordered two IMCO satin shorties, so I hope all goes well.

Re: Shorter drives?
 

No, I'm looking for another one.

Re: Shorter drives?
 

Well, here I had decided I didn't need to try them. After reading this, maybe I do need to give it a try.
I've got a 32' running surface, 25 degree V and I would guess about an 18" notch. I run labbed, 25" mirage+s. A friend has a set of labbed 26" Hydros I could probubly get for about what I could sell my Mirages for.
Maybe I need to start thinking about this again.

Setup on a lower unit is pretty simple, right? Just torque the verticle nut and set preload on the carrier nut? And get everything in the right place. :eek:

Re: Shorter drives?
 

Every application is going to be different - TESTING - is the only for sure way to know - I know on my application (35 Fountain single step) the RPM increased by 200 - up 2" & up 2" prop pitch netted me 5 MPH

Re: Shorter drives?
 

Here's a few pictures at eye level with propshaft and bottom of boat. Does it look like I'd be safe going up 2"?

Re: Shorter drives?
 

Sprague,

1) Does your boat have a notch in the transom?

2) Did you notice any increase in speed at your cruising range....let's say 3500-4000rpm?

Thanks

Re: Shorter drives?
 

Payton,
Gear lash has to be set up.. Pinion height and driven gear depth.. The Imco lowers are different than Merc cases. Well the number of shims are different to get the same pinion ht..

Dick

Re: Shorter drives?
 

Dick, how much to make the swap for me? If I simply bring you my lowers?

How much more if I watch? :D :D

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I just ordered my pair of Shorties, so we'll just see !! :drink:

Re: Shorter drives?
 

Kamma,
My Scarab does not have a notch. Straight Vee. I did not check the speeds at cruise. My guess is the cruise was up due the the Bravo 1 vs. the Mirage props.

Re: Shorter drives?
 

Ben, thanks for the response....I also forgot to ask you how many cubic inches are your engines, supercharged or naturally aspirated, and what kind of power are they putting out if you happen to know? Thanks again.

Re: Shorter drives?
 

Kaama,
At the end of the project, the engines were 505 ci @ 780 HP naturally aspirated. The top speed was 105 mph with Bravo I's 28p turning in (103 mph out) @ 6600 rpm. Add +2 mph to both with Herring 5-blades at the same 28p (Herrings - 200 rpm vs B-1's) I might have had slightly more MPH opportunity if I had Herring 27p to test with. At 9500 lbs. with 20-40 gal of fuel an two on board, this is a "hull efficiency" of 250...not bad for a 1987 straight vee.

Re: Shorter drives?
 

What you running as far as prop rotation ?
in or out ?

Re: Shorter drives?
 

BenP,

The reason why I have asked is because your set-up is very close to mine, only I am not running as much HP or RPM's as you. I have a 1990 32' Active Thunder with a conventional "V" hull (non-stepped hull), and no notch in the tansom. The way my drives are set up are with Bravo One XR's on Stellings extention boxes with labbed 32" Bravo One four blades. The drives are stock length cases and they sit about 3.5 to 4 inches (prop shaft centerline) below the hull. I have a pair of IMCO -2" shorties pretty much ready to bolt on. I know that every hull is different, but it would be nice if I could tack on another 5mph as well----but as I have always said; "we shall see".

Anyway, it sounds like you have a very nice running rig. Thanks for your feedback.

Re: Shorter drives?
 

Kaama,
My set up is also with Stellings extension boxes. Is the last 6-8' of your hull flat? Hook or rocker? This can be important for max top end. What length are your inside strakes in relation to your hulls CG? You may need do some fine tuning in length to get the attitude just right as you trade off some trim leverage with a high X. This is not a big deal... nothing money won't fix!