Stock Bravo lower - what speed is sportmaster style lower needed?
What is the breakpoint/speed for the Bravo with stock lower with regards to hydrodynamics? Do you pick up 1 or 2 mph with the crescent shaped lower unit? At what speed would you see the improvement? Is 90 mph the point when the standard lower losses hydrodynamic efficiency and a sport master would be a better lower unit, and a boat might gain 1 mph or 2 mph?.
Factors to consider--- in the present set-up the drives are mounted fairly high as is, an extender box will probably not be purchased anytime soon, but if one of outdrive (lower) fails, and needs a rebuild, well at that point maybe an IMCO lower would not be a bad purchase if we could gain 1 mph to 2 mph, or greater.
Who has experience with this, and if you changed just the lower form a stock to crescent shaped lower did you gain any mph? From what mph to mph?
i have put the imco/ speedmaster lowers on a couple boats hangin around the 90. 92 mph mark and have seen increase in speed. a single activator with a 500 hp , charged , picked up 3 with the sped master/ imco style standard length. and then 6 with the shorty version
also worked on a 38 fountain with messaged 502's. picked up 5 goin to the shorty.
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My boat runs 89.8 loaded,92.8 light (single engine 272 baja) with std length bravo with well blended hydro nose cone. When I bolt on my spare drive with dual water pick ups and no nose cone it barely breaks 85-86 mph,Smitty
i have put the imco/ speedmaster lowers on a couple boats hangin around the 90. 92 mph mark and have seen increase in speed. a single activator with a 500 hp , charged , picked up 3 with the sped master/ imco style standard length. and then 6 with the shorty version
also worked on a 38 fountain with messaged 502's. picked up 5 goin to the shorty.
Thanks Bo.
On a separate issue, I read some of your posts one time about water pressure with the low water pickups and added speed (near 90 mph). Keep up with the good advice/and real world experiences.
My boat runs 89.8 loaded,92.8 light (single engine 272 baja) with std length bravo with well blended hydro nose cone. When I bolt on my spare drive with dual water pick ups and no nose cone it barely breaks 85-86 mph,Smitty
Smitty,
Is your drive high (X)? What prop are you spinning? That nose cone is a hell of an improvement in mph, that is why is was thinking your drive may be deep, is it? That is one strong Baha @ 92.8 mph, that must surpirse the sh*t out of alot of people.
Is your drive high (X)? What prop are you spinning? That nose cone is a hell of an improvement in mph, that is why is was thinking your drive may be deep, is it? That is one strong Baha @ 92.8 mph, that must surpirse the sh*t out of alot of people.
My prop centerline is 7.5" below the pad (stock),I tried a stellings box last year in the plus 1" position and lost 3-5 mph and boat cavitated,blew out the prop,road terrible in rough water,bow steered and every other ill handling thing you could think of. I tried spacing drive back down 1" which slowed boat down further but helped it handle better,tried 1/2 dozen different props.
Before the stellings box I ran 89.5- 89.8 mph any day of the week with 30 pitch merc labbed 4 blade at 6000 rpm's max trimmed with a lot of slip, up to 4 passengers and coolers,3/4 tank of gas etc. Runs into the 92's with a best of 92.8 running light ( with 20-30 gallons of gas,2 passengers) 32 pitch merc 4 blade labbed by unknown source,turning 5700-5800 rpm's,tiny bit sluggish in midrange compared to 30 pitch.
After adding the box, boat would barely break 85-86 mph loaded,and 88-89 light if you really held it forever. Pulled box off and sold it out of pure frusatration (I had enough after a full season of prop testing,spacer testing and evil handling).
I have 2 bravo drives with full max worx xr upgrades,one that I run all the time with side water pick ups that has a very well blended hydro nose cone and one with a stock casing with dual water pick up that I run when the other one is broke and I'm waiting on parts. I have swapped from one drive to the other on numerous occasions over a weekend (out running fri or saturday and break something or chew up a set of lower gears),bolt spare drive on sat night and go back out sunday and their is night and day difference. Like I said,the boat easily hits mid 89's loaded,bolt the spare drive on and I'm lucky to break out of the the 85's in the 86's and I can't even think of turnig the 32 pitch even running light. When I back out of the gas with the drive with no nose cone I can feel the transom literally dragging and feels like the transom is lifting up a little which makes the boat bow heavy and bow steer a little which is spooky.Now the internet "experts" tell me the drive with nose cone should actually have more drag because of the surface area being greater and my prop slip should be less but I have experienced exactly the opposite,I lose 200 or so rpm's with the spare drive and a matching amount of speed. I have never gotten around to putting a nose cone on it as I have been meaning to try a shortie but am gun shy as to weather it will be a waste of money after all the problems I gained going up 1" with the stellings box last year.I am still freshening my motor and won't be back on the water for another few weeks,I'm hoping to see mid 90's with my expected hp gain but we all know that goes ,Smitty
My boat runs 89.8 loaded,92.8 light (single engine 272 baja) with std length bravo with well blended hydro nose cone. When I bolt on my spare drive with dual water pick ups and no nose cone it barely breaks 85-86 mph,Smitty
I'm interested on how you ultilize all that speed on the constant rough water? Or do you stick to the river?
I run all the boatable lakes in Michigan: higgins,torch,lk charlevoix,saginaw bay,saginaw river,houghton lake,hardy pond,lk huron,st claire river,lk st claire,lk Mi, plus I go out of state too sometimes. When it comes to rough water,it really depends on the spacing and wave/chop size and the load I have in my boat. My boat really suxx in true 3 plus foot waves and cross chop,most my high speed running is in smooth water to 2 footers,when I get into 3 footers if they are spaced right i can sometimes get on top of them running hard 50-70 mph with some positive trim with tabs dragging (they are longer than the stock bennets my boat came with),other times I just have to bury my bow somewhat a plow thru them bouncing around getting my bones jarred,3 footers can be accompanied by rogue waves where I have been air born out of control with nasty landings,my boat really isn't worth a crap in true 3-4 footers and I have had to return from places at 40 even 20 mph just getting pounded,any one with a boat my size that tells you different is usually over estimating the size of the waves they are running in,Smitty
Last edited by articfriends; 06-16-2007 at 11:17 AM.
Platinum Member
,Smitty
