A Few Questions

[Tony Davis]

Originally Posted by Gordon McMath

How do you know all this then lardy boy

Coz I listen more than I speak.

Bet your glad you've winterised your mates cat motors!!!

[Tony Davis]

Originally Posted by Peterjfinch

It´s been recommended, form another site, that on a twin inboard / outdrive setup, the anti cavitation plate should be level with the bottom of the hull (keel), It´s also been recommended that the prop shaft should be about 5 inches below the bottom of the hull.

Any opinions?

Pete

I'd go for +2" on the cav plates, but it really is just a guess, it'll certainly work, just as 0" would, Propshaft 5" below may also be ok, but I'd consider that a bit on the high side for your application.

[Peterjfinch]

Originally Posted by Tony Davis

I'd go for +2" on the cav plates, but it really is just a guess, it'll certainly work, just as 0" would, Propshaft 5" below may also be ok, but I'd consider that a bit on the high side for your application.

Thanks Tony, I guess that means, the cav plate 2" below?

Pete

[Tony Davis]

I'd go for 2" above the running surface.

[Peterjfinch]

Originally Posted by Tony Davis

I'd go for 2" above the running surface.

Ok, thanks. Should I also be trying to keep the engines as low as possible, in the hull? Is there a way to decide, the height of the drive and the height of the engines?

Thanks again.

Pete

[larby]

I'd guess the height of the engines is dictated by the height of the drive in order to keep the driveline straight and aligned, as with a bravo. One of the many reasons I wanted to change drive as my engine is 10" or so higher than it could be to get the crazy x dimension (height of the bullet) it requires.

I'd imagine the majority of people on here who would normally reply but didn't, did so because the x-dimension can be affected by so many factors it's hard to just guess a figure, and I wouldn't want to give the wrong advice!
It's easy to play with the odd inch up or down with our boats to fine tune, but I doubt many on here have had experience with setting an x dimension on a boat like this from scratch. TD would be the man on the mark on this one I'd imagine, he's the one I'd probably pester for an answer if I needed to know!

Personally, I'd find out what height the bullet was at with the original setup,and go for around that to start with, as that is what your hull is designed for. People run high x dimensions, even surface piercing these days, but that's because the newer hulls are designed to work with that.
If you're not sure of the drive length of an alpha versus a TRS I'm sure I could find out from OSO for you, or someone on here may know. I'd imagine a TRS is a fair bit longer than an Alpha though...

James

[Peterjfinch]

Originally Posted by larby

I'd guess the height of the engines is dictated by the height of the drive in order to keep the driveline straight and aligned, as with a bravo. One of the many reasons I wanted to change drive as my engine is 10" or so higher than it could be to get the crazy x dimension (height of the bullet) it requires.

I'd imagine the majority of people on here who would normally reply but didn't, did so because the x-dimension can be affected by so many factors it's hard to just guess a figure, and I wouldn't want to give the wrong advice!
It's easy to play with the odd inch up or down with our boats to fine tune, but I doubt many on here have had experience with setting an x dimension on a boat like this from scratch. TD would be the man on the mark on this one I'd imagine, he's the one I'd probably pester for an answer if I needed to know!

Personally, I'd find out what height the bullet was at with the original setup,and go for around that to start with, as that is what your hull is designed for. People run high x dimensions, even surface piercing these days, but that's because the newer hulls are designed to work with that.
If you're not sure of the drive length of an alpha versus a TRS I'm sure I could find out from OSO for you, or someone on here may know. I'd imagine a TRS is a fair bit longer than an Alpha though...

James

Thank You James. You have explained things very well. Between yourself and Tony, I´m sure that I will get the right advise. I am trying to find out the difference, with the length, from centre of drive shaft, to crntre of prop shaft, between the Alpha and the TRS. Anything anyone can let us know, will help.

I don´t know enough to make an inteligent guess but I keep thinking that keeping the engines low, in the hull, will help all round. Although the TRS drives are my only option, at the moment, I´m not going to cut the transom and end up with a pair of props, too low in the water. I hope you can understand my concerns.

All ideas are welcome.

Pete

[larby]

Right then, I've managed to find that a TRS is 23.122" and an Alpha/Bravo is 21.81".

Basically we can say that the TRS is an inch longer than the alpha or bravo. All you would need to do now is mark where your input would have been horizontally on your hull (where the driveshaft goes out through the transom), mark 1" up from that, then centre the input for the TRS to that upper line and draw out your template. That is assuming you want to keep the same x-dimension. Tony would know better than me what you want to do there.
It's also worth noting that the TRS will probably position the prop further aft from the boat. As the prop moves aft (has more setback), you need to also bring it up slightly to have the same equivilent x dimension. For the sake of argument, say you had 0 setback and 7" depth, if you went to 6" setback, you'd want to come up an inch to 6" depth to compensate. For this reason I'd probably knock 1/2" or so off the height to allow for the extra setback from the TRS drive.

Alternatively, if you want to go with propshaft a set distance, say 7", below the bottom, then the point where your propshaft comes out on your template wants to be 15" up from the bottom to give 22" total, and so on. Knowing where that driveshaft lays will also then enable you to work out where your engines will be, as this height will also be the height of the crankshaft.

Hope that helps again, and I hope I'm right with what I say,

Cheers, James

[larby]

Oh, as for centre of gravity, yes it'd be best to have the motors as low as possible, but in your application the correct depth for the drive is more important and will dictate where the engines have to go.

James

[Peterjfinch]

Thanks James.

That makes things much clearer, thank you. I took out the original front engine mount crossmember today. Luck I didn´t run her with the original engines, I think I might have had to use my diving skills, to find the boat.

The crossmember bolts were so badly corroded, they snapped the first turn of the spanner. There was about 3/16", or less, left of the 1/2"bolts. Had to split the nuts on 3, of the other 4 fixings. The box section had disappeared on one side and only where it was covered with fibreglass, was there any good metal.

Will take some photos tomorrow and show you.

So, apart from the recent questions, including "where to put the through hull water scoops". I now need some advise on a new mounting for the front of these engines. When They were lifted into my place, they weighed 564KG, wet and with the velvet box on. There are some stainless pipes to add to that weight but not much. Any ideas for a new mounting would be welcome.

Tomorrow I will post some photos of the engine bay, to get some ideas.

Thanks for all your help.

Pete

[Peterjfinch]

Hi,

As you can see, from the photos, the crossmember was not in a good condition. In one photo you can see some of the casualties from the removal.

Also, you can see the options I have, for the through hull scoops. My idea is to keep them as near to the transom as possible and secure the valves to the transom. I have added a photo of the scoop and valve.

Any suggestions for the new front mountings? Another beam, or something different?

Thanks

Pete

[mattboy]

you have been busy mate. looking a bit clearer in the engine bay now.
Didnt think that crossmember looked much good, i see the more stuff you removed it became apparent.

speak soon
Matt

[Peterjfinch]

Originally Posted by mattboy

you have been busy mate. looking a bit clearer in the engine bay now.
Didnt think that crossmember looked much good, i see the more stuff you removed it became apparent.

speak soon
Matt

Hi Matt,

Still not sure about the water pickup position though. It has been suggested that 12" in front of transom is ok but no confirmation as to why. As you can see, without ripping more floor out and for good support from the transom, as near to the transom as possible would be better for my situation. The only thing is, as I´ve not had any answers on this question I am concerned that being close to the transom could be a problem. I want to cut those holes out, ASAP, so that I can get busy with the seriously soapy water.

Yes, it did become more apparent, as soon as I started on the bolts. I know it did look rough but was not expecting it to fall apart like that. Anyway, better out than in. Not exactly the lightest way to support two engines. Maybe I should start a "Engine mount design" thread and see what the people "in the know" suggest.

Even clearer in the engine bay now. Everything out, including all wiring cables and plumbing. Also started to remove the interior, on the drivers side, to check that there are no cable splices, on the way. All looks good and all inside trunking. That will make the re wire easier. Will also give me an idea for the re upholstery.

I´m getting a list together, for supplies and trades, so will be asking the normal questions soon.

Speak soon

Pete

[Tony Davis]

12" from transom is where I fit them, but it's your baby, so feel free to stick em where you like, 1" would be just as good, or even transom mounted similar to a speedo pitot, there's no hard and fast rule on a non stepped running surface!

Here's where we fitted it on Gaylord

[Tony Davis]

Mercruiser used to do a twin version of these, with a pitot as well, fitted dead centre of the transom, right at the V. Lovely bit of kit, I think Tony Hamilton may have one for sale!

[Peterjfinch]

Originally Posted by Tony Davis

12" from transom is where I fit them, but it's your baby, so feel free to stick em where you like, 1" would be just as good, or even transom mounted similar to a speedo pitot, there's no hard and fast rule on a non stepped running surface!

Here's where we fitted it on Gaylord

Thanks Tony,

I thought that nobody was saying anything because of some problem with mounting them nearer the transom. I was concerned with any weakening, caused by putting two holes through the hull, close to the transom.

I would prefer internals but agree, the external do look good.

Thanks again, holes in the hull, soon.

Pete

[Peterjfinch]

Any ideas for a new front engine mount? Please.

Pete

[Tony Davis]

I'd remake what you removed, it obviously did the job for a number of years.

[Peterjfinch]

Originally Posted by Tony Davis

I'd remake what you removed, it obviously did the job for a number of years.

Good point Tony. I was just concerned that the new engines are a bit heavier and will be further forward. also, the one I removed was also very heavy.

Would I be better to spread the weight over both steps and use a lighter material?

Pete

[Peterjfinch]

Originally Posted by Peterjfinch

Good point Tony. I was just concerned that the new engines are a bit heavier and will be further forward. also, the one I removed was also very heavy.

Would I be better to spread the weight over both steps and use a lighter material?

Pete

Anybody else got any ideas???