Looks Like I'ma gonna be a Caddy Guy - Now What?

Automotive powered airboat discussion.
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Slidin Gator
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Re: Looks Like I'ma gonna be a Caddy Guy - Now What?

Post by Slidin Gator »

OneBFC wrote:
Sun Apr 25, 2021 10:02 am
Good luck with your new project!
Thanks BFC, this boat will be light and I should be well under the 2:1 weight to thrust ratio, don't think I'll get to 1:1 helicopter capability though :cry:. I'll be cutting a bit deeper, it's a 17-1/2" transom and I plan to make it 9" high. I'm curious if you ever ran the same engine/prop setup between the open hull and deck over setup to compare performance? My son and I can swap props between my 300 Hp 540 on a deck over and his 250 on the open and we use identical pitch settings for the same RPM. One buddy tried to tell me that the 300 must not be making power, but that is not right, she puts out. It's the cleaner airflow into the prop letting her grab.

I plan to integrate the radiator into the seat rigging with electric fans and ducting to keep airflow clean. This will keep me at lower RPM and should help with torque roll. Other advantages will be lower sound signature, faster warm up/cool down and move weight forward. I have been using electric fans in my buggy for years and have had great success.
One Eyed Gator wrote:
Mon Apr 26, 2021 10:08 am
... to me each blade has a tip speed it likes for best all-round performance. 76 vs an 80" prop at the same engine rpm have a very different tip speed. Keeping the engine RPM about the same to get best power out of the engine and using the gearbox ratio to dial in the tip speed while also reducing to load of the engine.

Where I tend to agree is a motor with plenty of torque doesn't seems to feel the prop load as much. Specially a smaller diameter need for a deckover to keep the motor as low as possible
I also know that on a round bottom hull and big prop and low gear has quit the pucker factor of torque roll. lol
Just my thoughts
1 Eye, we are on the same page, understand the tip speed issues and variations. Torque roll is my #1 concern here, I don't want to go all stupid on it and end up with a boat that is not easy on the Capt.

My reference point is 14' Palm Beach hulls built for this motor. Those boats are 4-6" wider (7'-6", 90") to run super wides on 2.68 gear. Torque roll is a bit complex. Starting from a stop in water it is strictly a function of flotation. Once the boat is moving, hydro dynamic lift come into play. On ground it's the outer rail pushing the most.

The worst case is pure flotation from a stop. In this case the extra 6" of hull width increases flotation and the moment arm resisting roll. Torque Roll resistance is "roughly" equivalent to width^2. To match torque roll the comparison is (84/90)^2 = 87% maximum torque. Assuming a 600+ motor the maximum gear equates to 2.68 * 87% = 2.33.

So that's where I am, somewhere between 2:1 to 2.38:1 gear and matching prop, somewhere between 76-79".
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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OneBFC
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Re: Looks Like I'ma gonna be a Caddy Guy - Now What?

Post by OneBFC »

I think you will hit 1:1 ratio. We are right at that with ours and the LS makes more power. The 80 inch max i turn put out 1435lbs thrust. The same prop on a 418ls put out 1570lbs. I think the LS needed a 4th blade because we had 15 degrees in the prop and the Max blade has a narrow happy range.

Love the prop though!
-Russ
-----------------------------------
The only thing stopping you is FEAR
400+hp Ecotec, 12x7.6 DBDO, 80" 3B Maximus, 2.3 OX,85+mph, water = purely optional
Life begins at 2 BAR, Just a good ole boy
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Slidin Gator
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Re: Looks Like I'ma gonna be a Caddy Guy - Now What?

Post by Slidin Gator »

1570 is not gonna do it for 1:1, maybe a different baseline.

Below is the weight budget from the boat this is going on based on weighing everything that came off/out of it. Full up O-540 with headers and prop hub is 438 lbs. So I'm adding 100 lbs of motor/gear/radiator. The 340 lbs of seat and cage rigging was outrageous (Includes mufflers), giving me a maybe 100 lbs less total with light rigging, heavier engine/gear combo and 3/8" vs. 1/4" poly.

Image

Image

There is some weight to take out of the 158 lbs of crap, but it's gonna be around 2,400 lbs or more loaded, no hovering.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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Slidin Gator
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Re: Looks Like I'ma gonna be a Caddy Guy - Now What?

Post by Slidin Gator »

Did some searching on part numbers today, this turns out to be the most significant. 19329865 comes right up for a GM crate L92 engine, the 2016 manufacture date confirms I got a real good deal at $2,648 delivered to the house. :cheers:

I felt guilty taking this engine apart, but I got over it and now have a low mileage factory rotating group and 30 lb injectors up for sale.

Image

Dropped off the block and heads at the machine shop today for inspection, it's not gonna take much cleaning.
I still think I-10 is the Mason Dixon line.
2013 Mike Stossel Boats, 13' x 7' deck over, IO-540, 74" NGQ less winglets.
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