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Rigging question for new hull

John C

Well-known member
Last week I almost put a stump through my fiberglass hull, so I made the decision to go to a alum. hull with poly. I can't use my rigging that I have now because it is sitting on 4 " stringers. If I put my rigging into an alum. hull it will sit 2" lower. The transom on the alum hull is approximately 4" higher than my fiberglass hull. So if I set my rigging in the alum. hull the tip of my prop would extend down into the hull an additional 6". I already 5" below the transom now. That would be 11" of prop in the boat. So I think my old rigging is out of the question. If I have new metal works built How do I determine how high to mount the motor? I have been told that it is important to keep the center of gravity low, especially with a heavy motor (520 Caddy with gearbox). But then I have also been told that you should not have the tip of your prop dip down below the transom for the best trhust/performance Maybe some of the rigging guys will chime in and help me make a decision.
I may not be the prefect person to answer but have a little knowledge that may help.
Your CG in any boat is important when designing you always want your CG as low as possible with a airboat you are defying that rule to start with that is one reason for the wide hulls. To figure out where you need the engine mounted figure where the center of your prop shaft has to be. then measure your hull, engine and reduction unit. In my mind the best hing I have seen is the Long belt reduction for the CG purposes but they don't seem to be that popular
If you need to make your old rigging work I can help you out if you get the extra metal.. If you need help measure to give the builder some figures let me know and I will swing down there and see if I can help you out

speaking of metal , you have choices if you build new steal is cheap and strong but heavy and rust , Conduct tube for the cage but I have not seen any that don't rust and break fairly quick , but it's cheap, then there is but Moly steal very strong and expensive , stainless steal isn't cheap but it doesn't rust out either and Aluminum is a good choice it's light, doesn't rust etc.. you could mix and match for example a stainless steal engine stand and aluminum cage and seat stand
gh, thanks for the offer. The long belt drives are heavy and expensive. I guess the question I need answered is, if you were to build a new set of metal works how much prop should go below the transom.?I think I am going to use d.o.m. for material. PM me your PH#.
if I was building new I would not plan to put any prop below the transom, that gear box you have should give you 10-12" I was just speak ideal but in you case I would use what you have as far as that goes.. You have a nice prop and gear box I would suggest you measure on those items for your figures maybe give your self a little extra room in case you ever do want to increase prop diameter

I'll Pm you cell
something you might try is making an adapter to fit your rigging into an aluminun hull,i did this a few years ago, it was a combee rigging into a panther hull, i was able to set tha height where i wanted it. that boat is still out there today i sold it to a guy in arkansa who rented it to hollywood to make tha movie roadhouse 2 ,i set it up for bowfishing its tha big green camo panther boat with tha raised deck and lights
you don't want the prop no more than 5" below the transom most are 2 to 3 in that keeps it to where you can still blow the back wash out and help keep it from going over.
for metal works you can use almost any thing but it really depends on the type of running you do as to what will last the longest.
c chardt, this is going to be on a 15x8 alum. hull, high sides. I can run the 15x8 hull I have now and turn the key off and not have water come over the transom. Would you still recommend the prop be 3" below the transom? Since I will be starting from scratch, I would like to get it rite the first time. I do not know, or have any info on how much it disrupts your air flow by having your prop 3" down in the boat. It might not even be noticeable. I know that you re-rig boats. Have you ever re-rigged a boat that had the prop below the transom line and then set it up so that the prop was flush with the transom? If so could you notice any difference in thrust or sound? Or is it more important to keep the CG low.
I had my old fiber glass hull it was a direct drive 350 and I had the prop about 10 or 11 in below the transom and the boat still ran good but I knew i was putting a gear box on it and that raised it back up and could not tell my boat i run now the prop was 4 in in side the hull and then I put a different prop on now it's level with the transom and it still works good I could not tell any difference. but when it gets re rigged it will be back to 4 or 5 in down in side. they say that the center if gravity will change how it handles but I think it's fine the way it is won't know until I re rig it.
A boat that is hiding too much prop will also be louder. The prop tips bounce shock waves off of the transom as they pass it.
One of the things that helped Thunder's boat run so quiet is that it was a deckover.

This metal works is set up so the prop will be 4" below the transom
c chardt, fanman said that he made an adapter for his rigging to fit into his alum. hull. Have you ever made one, or extended a set of metal works?
No but I have thought about it before It would be easy to do It will just add a little weight but should work fine