Grandsport setup questions/advice

[Punteh]

Hey everyone,

I’m looking to make some changes to my Grandsport to get more top end out of it. The current setup is more or less as it was when it was bought in 1990. It has a Yamaha 200 and an Allison hydraulic jackplate.

I’ve added a low water pickup nose cone and an Allison cavitation plate. I also have an Allison skid planer but I don’t see a way to mount it with the brackets with the jackplate I currently have.

The prop I generally run is a 30”. I have to re-measure when I take the boat out of the lake in the coming weeks but if I remember right my prop shaft height is about 1.5-2” below the pad. I have one more set of holes on the jackplate that I could raise the engine but that’s it.

Based on what I’ve seen other people saying online I think one of my biggest issues is with prop slip. Using the Mercury calculator my slip is around 30%. On GPS I’m basically maxed out at 75 mph.

My thought is to get a new jackplate with more setback (likely a Bob’s) as my current setback is 5.5”. What I need advice on is what size to get. The motor weighs in around 400lbs and I’m generally around 200.

Would a 10” setback jackplate be a good place to start? Or would that be too much? Not enough? Would moving the engine back help reduce the prop slip I’m getting?

Any other advice would be great too.

[RiverRat71]

My GS had 2.5XS which weighed in at about 440 & had a factory 8" Allison plate. I ran it about a 1/2 below the pad. With my Hydromotive 28 pitch 4 blade at 6800 & 86 mph slip number was 5 percent with the 2.1 Torquemaster. I think you would be about right with 10" setback with the 400 pound Yami. You def need to bring the motor up to around 1/2 below. I'm not a guru with the Grandsport but that setup ran good for me. RR

[aj06bolt12r]

Your biggest problem is that the propshaft is way to low. Allisons do not like to run with the prop buried and I don't know exactly what prop you have but most high performance 30p props are designed to be surfaced if you run them buried they will not perform to their potential. Assuming your boat still has the stock lip you will want your propshaft somewhere between 1/2" below to even with the pad. If your motor weighs 400lbs I'm going to say that you will want 10-12" of setback. I like the hydrodynamics jackplates. Easy adjustment and good quality.

Also once you get the setback and prop to pad height in line your speeds should pick up drastically if that Yamaha is in good running condition. You will want to make sure that your steering is tight and in perfect condition and you will want solid motor mounts on that puppy or its going to be much more difficult to drive than it has to be.

Obviously you will need a good nose cone on that lower unit with low water pickups or you will have no water pressure running the propshaft up near even with the pad.

[David]

My brother and his wife have a Grandsport with a 150 HO Evinrude Etec. That ‘s probably 30-40 lb more than your motor. They have a manual plate with 10” of setback and run a 24 Raker. 74 mph on the rev limiter, and the slip calculation shows negative. Negative slip is impossible, but I believe their actual slip number is low. GPS is consistent so probably correct, the engine definitely has a limiter, the prop could be miss labeled.

My point is that slip can be interesting, but it’s speed you want. And 75 is low for a Grandsport with 200 HP. There are 200 HP Grandsports on the Allison Owners site that claim over 90 mph.

Go up slowly on the engine height and watch your water pressure. My SS gets faster with height. Your GS will as well. But Allison’s will blow out and can hook, so go up slowly and when the speed returns a minimal or zero, stop going up.

[JR IN JAX]

I have the 300XS with a CM hydraulic plate and I run my gearcase even/slightly above. When the GS gearcase is too low a lot of speed is scrubbed off and the boat rocks a lot. I am sure my boat is not set up optimum but I have so many hours of seat time, I don't notice. My prop is a 34 Promax the was intensely modified by Merc Racing [ a Frankenstein] and is a struggle to get on a plane but can cruise at close to 90 [6K] and gets around 6 MPG. Since it is not perfect, it can be scary [which I have always liked].

[David]

JR, does your boat have the stock lip?

[whipper]

10 or 12 inches of set back is normal with a 2.5 some go 14 for top end . The more weight the less setback you need to match the boats designed fulcrum point intended for a 2.5 at 340 to 360lbs.You never mentioned what gear case and ratio your running? You will want to set it up to run no lower than 1/4 inch below the pad for top end. Easiest thing to do, would since you have a plate that can go up one more hole go up one more and try it for sure. That should get you to around 1/4 below but I think you need to be at least 8 inches of setback if not 10 at 400 lbs for top end.? You also never mentioned what 30 prop your running and what your max rpm is? You will free up alot of rpm running higher for sure. What's your max rpm. With a 200 setup right you should see 80+ easy. So two setups. Current one raise up that one more hole and since your only at 5.5 inches of setback try and move 40 lbs forward. Battery, lead shot what ever 40 to even 50 lbs forward should help. One of your offset weights put it as far forward as you can. Under the bow there's a hole behind the glove box stuff it down there and try. Not all props are created equal so there's also that. 28 Drag 3 and 4s work great I know. Second setup get a min 8 inch setback run 1/4 below and run a 25 trophy to see what rpm you can get out her. Then go up in pitch and speed until you can't reach max rpm any more and decide on weather a 30 might be to steep, also could be a problem. {gear ratio + Limiter}?? 8 to 10 setback is were I think it should be and for sure 1/4 bellow and above to even. I run even and above mostly and 1/4 below with 4 blades. Cleavers even to 1/4 above because they are surfacing not just semi. thats all I got. Hope something helps because a GS running 75 ain't right. :] LOL

[Punteh]

Whipper thanks for the reply and the info.

The gear case is the stock Yamaha case with a low water pickup nose cone added. The gear ratio is 1.86. Max rpm according to the tach is about 6900.

Here’s the prop I usually run. Still trying to figure out myself what differentiates and identifies what types of prop so I’ll just show you what I have (don’t know why the pictures keep posting sideways):


I just pulled the boat out of the lake this afternoon and just did a quick look at a few things and it turns out that I don’t have the option to go up another hole on the current setup. There’s a free set of holes on the plate for the top motor mounting bolts but no matching holes for the bottom. I’ll measure my prop shaft to pad height in the coming days but eyeballing it I’m figuring it’s at least an inch below the pad. And unless I missed something that’s as high as I can get with the current setup.

So my plan is to get a 10” Bob’s hydraulic jack plate. What things should I be aware of when replacing a jack plate on a GS? I see I have to move or remove the gas tank to get at the mounting bolts for the plate. Do all grand sports have reinforced transoms or would I need to get some sort of reinforcement to put inside when I replace the jack plate?

[aj06bolt12r]

I didn’t know what that prop is.

But I can tell you that you don’t need to remove the gas tank. Allison welds the heads of the bolts to a plate and then screws that to the inside of the transom. The bolts won’t turn on you. You can just remove the nuts and pull the plate.

You also don’t need to worry about reinforcing the transom. It already has metal internal bracing. It’s plenty strong.

Careful w the hydraulic plate. Make sure you know exactly how high the propshaft is at all times. Allison’s drive awesome with the prop a little too high.. Until they throw you into the water.

[Punteh]

I didn’t know what that prop is.

But I can tell you that you don’t need to remove the gas tank. Allison welds the heads of the bolts to a plate and then screws that to the inside of the transom. The bolts won’t turn on you. You can just remove the nuts and pull the plate.

You also don’t need to worry about reinforcing the transom. It already has metal internal bracing. It’s plenty strong.

Careful w the hydraulic plate. Make sure you know exactly how high the propshaft is at all times. Allison’s drive awesome with the prop a little too high.. Until they throw you into the water.

Have they always welded the bolts in place? This is a 1990 or 91 model.

I’ll be getting a gauge with the jack plate so I’ll know how high the prop is at all times

[David]

There was a bolt recall on the early Grandsports. You should confirm that you have the improved bolts.

[RiverRat71]

Have they always welded the bolts in place? This is a 1990 or 91 model.

I’ll be getting a gauge with the jack plate so I’ll know how high the prop is at all times

I think the bolts were always "captured" but like David said they changed some things on the early GS transoms. Call & talk to Bill at Allison. He will tell you everything you need to know about your boat. RR

[JR IN JAX]

I took a little off of it, but not quite flat.

[whipper]

1/4 inch below the pad. Can you just add the OS to the existing transom plate since they are both bobs? Make sure you have the correct lifting rin to hang your motor while you do the plate. I just use a come along on a 4x4 in the rafters in the garage works great. That 30 pitch your using is a semi surfacing style. They perform real pooly if not half in half out of the water. Thats were the 1/4 bellow the pad setup comes into play. If your used to only this currant setup its going to be a real shock how much higher off the water you'll be. Way more care is going to have to be taken with tirm. Use a GPS. After 65 and 70+ if you bump the trim and you dont pickup .01 mph at least back down once. lol Neutral to slightly neg is proper trim on the pad at wot with a 225 pm for example. Just saying use caution she is going to perform alot different than before. haha For the better but is going to take getting used to. With 1.87 gears and a cone a 28 drag 3 by performance in stock form or later just tip cup added is almost as good as it gets. They look alot like your 30. But with 200hp a 28 will give you all round better performance. Also cant go wrong with a 25 trophy at 1/4 bellow. When its rough out there a blast to stab a few hundred yards of furry until the next wake boat wake. haha Cant wait to hear your reaction after your test driven. :] Its also a good idea to add balls in that 10 inch cavity. Like little kids bouncy type and blow them up in there. It keeps you more buoyant at rest and thus less squat getting on plane . :] Cheers