repowering my 1976 hawiian jet boat.

[jroach'sflyinhawiian]

i just aquired a gutted hull of what was a 1976 hawiian jet boat. fiberglass all seems to be in good shape and the price was right so i am pretty pleased with the purchase, i will need to power it, and i am wanting to convert to either a stern drive or outboard due to the high cost of jet drives and needing to build an engine for it. i am leaning heavily towards running two or three medium sized outboards, my question is what is the best way to mount outboard engines to a previously inboard powered boat? yes i know that the transom will need significant reinforcement. the way i see it my options are (A) jackplates. (B) a fixed bracket attached to the transom. or (C) attaching my engines directly to the transom what would you do?

 

[Ned L]

Hmm, You are going to really be changing the boat. 1) you will be moving all that weight from about 3 feet forward of the transom to at least 18 inches aft of the transom (BIG move), and 2) you will be changing the transom from something that was designed for really very little strength to one that needs to hold up to hundreds of HP of thrust & torque.

 

[Scott Danforth]

Welcome aboard.

As Ned pointed out, you will be changing the structural requirements of the hull significantly as well as the character of the hull. To do it right, you are looking at a complete redesign of the structure and aft of the hull.

I would leave it a jet drive

which engine did it have? BBC, BBO, or BBF?

If I recall, most were BBC's not sure of the pump, Dominators IIRC

Jet boats are the one boat that hot-rodding parts work well on. a quick Craigslist search will net you a reasonably priced 468 and a rebuilt pump and your off burning fuel.

 

[jroach'sflyinhawiian]

was a bbf with berkley drive. i have NO jet drive or engine parts for this boat. i can not afford to go that route. im looking to power this boat for 1500 or less, would like to keep hull mods to under 1000. i realize that moving the weight back, can affect things, but a BBF and jet jet drive has to weigh at least 750 pounds fully dressed. a pair of 2 cycle 90/110 horse outboards cant be more than 500 right?

 

[jroach'sflyinhawiian]

transom is 1 inch ply wood i would think i could make it quite strong with very little effort.

 

[java230]

Mfg's list their outboard weights. I dont think you will get a pair that big for 500lbs. Can you even find a pair for $1500?!

 

[H20Rat]

Your budget goals are WAY short of what they need to be!! I wouldn't want to hang a 100hp outboard that I buy for $750 on the back of that... Might need 3, chances are one will run for awhile at least.

 

[jroach'sflyinhawiian]

i have found one for 900. more than likely i will wait to buy until winter, prices on non seasonal stuff seems to drop off here in michigan. are you saying two out boards of the size im looking for would be too small? or just that i wont find good running used engines.

 

[jroach'sflyinhawiian]

no one has any thing to say about the best way to mount my engines?

 

[greenbush future]

Twins are gonna be tough to match up, sync and all that. I have a factory twin set up and I'm struggling to sync identical outboards designed to run together. I would just bet the chances of you finding 2 of the exact outboards, and controls and sync set up will be a tall order. Throw some pictures up of what you have, my guess is your best and cheapest bet will be keeping her as a jet.

 

[redneck joe]

no one has any thing to say about the best way to mount my engines?

Not sure anyone is sold on changing the power, but if you do decide to run down the outboard road a transom mount I would think would be the easiest and strongest option, once you beef up the transom. That might come in at the $1000 hull budget you have pretty sure the engines will be tough to stay at budget. What size is the boat? Only Hawaiians I've seen seemed to be about 20' and if I remember they are heavy boats would 200 hp (total) be enough?




rj

 

[Bob_VT]

Welcome to iboats. What is your level of boating experience? Your idea of two or thee outboards is based on what? If you are trying to "match" the HP based on an engine coupled to a jet drive you are doing it wrong. The USCG has design standards and formulas for all types of boats and propulsion. You can not compare HP from system to system.

How big is this boat? I hope you are in love with the hull and design because designing the proper bracket to hang an outboard off of or even modifying the transom to take an outboard will not only cost significant money (I am not even counting a motor yet) but there are also many design factors to consider. There will have to be a new steering system, controls and you said the hull was bare so seats etc. The jet boats has short stringers and a keel stringer...... if you modify it the strings need to become full length since the motor also acted as a strengthening brace.

The Hawaiian was normally powered by a big-block 460 Ford motor. The motor was problematic and Ford got away from the marine business. The hulls were not too thick and tended to flex which let to string and keel problems with many. They were "go fast" in a straight line boat on smooth water with very questionable low build quality which was common. It is basically a Tahiti hull design but the company went belly up due to design lawsuits.

If you consider yourself a rookie I would say to move away from the boat and look for something you can enjoy now. You are looking at allot of work.

Now to properly answer your engine questions. You mentioned "no one has any thing to say about the best way to mount my engines?" Well without knowing which Hawaiian Boat model, length and type of engine......... there is no answer.

Get a photobucket account, take some pictures and post them here. This will give everyone a good idea on suggested answers. I would also drill some holes in that transom because you need to know whats inside it?

 

[jroach'sflyinhawiian]

thanks bob. it s a 20 foot boat. i have another boat ( sterndrive) that i just did a stringer job on. so the hawiian is not my first endeavor into hull work. i have never messed with any outboard engines before, i just thought it would be a cool direction to go with the project, as it is just that a project to work on when i get my other ones done. you mention the difficulty of syncing the motors, that is something i had thought might pose some problems. at this point i am just trying to gather information to make the right decision. there are two big reasons that i am leaning towards an outboard setup of some kind. one being simplicity and cost. the other being the added deck space of an out board engine, as i am planning on making this more of a general use type of boat and less of a strictly go fast boat.
​i had checked into a stern drive with a marine salvage yard local to me. he said he could get me into an alpha drive for around 1500. but then i would be losing some of the extra space im after and would most likely be another 2-3k into a sbc engine in the power range i would need. the same guy, had an old pair of johnson 3.6 outboards, and suggested that as a possibility. but thats a big motor and im not sure i want to go THAT fast. i would be more than completely satisfied with a 60-70 mph boat.

 

[jroach'sflyinhawiian]

after doing some searching on craigslist and some more research, i think i have decided to go with a stern drive. with my goals of turning my go fast boat into a more versatile vessel that i can cruise with or scream with, i think that is the best option to best utilize the layout of the boat without drastically changing the handling of the boat. i have a line on a cheap parts boat with a mercruiser stern drive that had a 140 hp four cylinder. will this drive be able to handle the torque of a mild sbc? maybe 300-350 hp? i do not know which model out drive it has, as i have not seen it and the current owner didn't know.

 

[500dollar744ti]

the drive on a 140hp mercruiser is good to about 300hp. I've got 270hp on one and I've broken a couple of gear sets (on used drives) from the torque but once I got it dialed in and reliable lower unit, it's been okay.

But I would say yes it could handle a mild SBC, people do it fairly often with those drives, they do break though.

The other option would be to mount the engine for a strait inboard configuration.

I'd say the boat would be comfortably fast with just the 140hp sterndrive in it and you would benefit from the lighter load at the stern.

You will also need to swap in a v8 upper drive to change the gear ratio.

 

[jroach'sflyinhawiian]

i know about the gear ratio differences between drives. just wasn't sure about any durability issues. i assume i willl probably have to re-prop also. but thats still a long way off. i will have to get some pictures up and start a project thread.