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Old 10-21-2009, 10:28 AM
TRBenj TRBenj is offline
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Originally Posted by OLNOLAN
Hey TRB, I think you're a pretty sharp dude but, maybe you need to do the math on prop changes. Okay, first I agree there is no substitute for cubic inches, the bigger the better. I also agree that more CI will help your rect. port heads act smaller in respect to boosting torque. You need to think in terms of torque in respect to boat engines rather than HP. A boat does not overcome it's mass like something rolling on wheels. Once you get a car moving it takes less torque to keep it moving faster and faster till you run out of gear ratio or HP. A boat on the other hand really never overcomes its mass.
Getting up on plane is just the action of rolling over the barrel of water that forms under the boat when you're trying to get it moving. Once you get the boat on top of the water, the drag is more constant. If you trim up, you get less boat it the water and less drag. Do you have the full size trim plate on the transom, is it fixed, adjustable or what?
As far as more RPM are concerned, it does not do you any good to prop for more RPM if you're decreasing pitch to get there. You did say a smaller prop,
which leads to the math on prop changes. You said you had a 13x14 and a 12.5x15, these are super pulling props w/1:1 drive but if you want more speed you gotta go up in pitch as in 16",17",18" to gain speed. Every inch you go up in pitch is gonna bring the revs down but what you're hunting for is the sweet spot in the torque that can still turn the prop at a speed desired. In your case, you can probably still go up in prop pitch, gain some speed and not give up too much low end pulling grunt.
If you still want to give up some cash and build a stroker motor, cool. But you can likely gain a little speed with a prop change, still pull skiers and only be out of the cost of a new prop.
Examples; 14" @ 5000 w/20% slip=53mph
15" @ same=56.8
16 @ same=60.6
19 @ same=72.0mph
You gotta see what it will spin up to with different props.
example; 19"@4600 w/20% slip=66.2
Food for thought, speaking of that, its supper time, later.olnolan
Olnolan, I appreciate the input. Maybe I should have prefaced this thread by saying that I have a pretty good understanding of how to make these boats fast with respect to engine powerbands and prop sizes. Ive done a LOT of experimenting with props on my other (similar) hulls so I know what works and what doesnt. What I lack is the BBC knowledge and how to end up with an engine that will give me the desired power in the 5500-5800 range. Just trying to figure out the best way to get there.

Just to clear things up, this is a 1:1 fixed inboard, so no trim. No trim plate either. This is actually a pretty decent sized v-hull, and despite the tank-like handling and relatively heavy weight with the big block, seems to be a pretty fast hull. The lifting strakes get a decent amount of boat out of the water. Regardless, theres no adjustment here.

I do disagree with your theories on props- whether that works on the types of boats youre familiar with I have no idea, but it doesnt work on ski boats. In my experience, maximizing top speed is a matching game between the hp curve of the engine and the size of the prop. Whatever size prop allows the engine to turn at or near its hp peak at wide open throttle should maximize top speed- other constants like rake, number of blades, etc held constant.

While the top speeds with the 2 props Ive tried are very similar, the larger pitch (12.5x15) had a significantly worse holeshot. I suspect that propping any larger than this would make the boat unskiable, and would likely hurt top end as well since the maximum RPM's would end up lower than 4900, and everything else is telling me that these heads want to breathe more than that.

Conversely, I could prop down to a smaller pitch (13x12) and pick up RPM at WOT and that would help holeshot. If my cam better matched my heads and allowed the engine to make more power (up to, say, 6500) then I would expect to see an increase in top speed by allowing the engine to turn closer to its hp peak at WOT. I just dont want to turn significantly more RPM's at skiing speeds than we do now (due to increased fuel usage and reliability concerns).

In other words, Ive seen smaller (pitch) props make a boat faster, and larger (pitch) props slow a boat down. There is no set formula on how to make one of these boats faster- its a careful balancing act. Generally speaking, when making engine modifications, sticking with a stock-ish sized prop will give you the best performance when doing top end (breathing) mods that add hp- you just spin that prop more RPM. On the other hand, going up in pitch will improve performance when doing bottom end mods that increase cubic inches (stroker) that increases the engine's torque.

In any case, based on factory settings and numerous other accounts, combined with how we intend to use the boat, I am confident that the props I currently have will be pretty close to what I end up using. I may dial in the cup a bit to tweak WOT RPM's, but for the most part, Im there.

The next step is going to be to determine our cam specs (at least measure the valve lift) and check out the pistons to try to determine what we have for a CR. Unless someone here thinks that we could pick up power by carefully tuning either (or both) of these, then I figure we probably have 2 options. A) go to a 496 stroker and take better advantage of the 990's, or B) Swap out the rectangles for some large ovals. Either will likely be accompanied by an appropriate cam change, possibly a CR change as well. If we choose the latter, maybe Ill hang on to the 990's and start planning a ground up stroker build for the future.
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