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New Cummins/Mercruiser Project

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Old 11-04-2008, 08:46 PM
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obrien,

I have to debate your remark about larger dia.

People have 43 Nortechs, 42 Cigs with Bravo drives on them, what dia props do they run?

Why when someone says diesel do people automatically jump to "larger dia"?

Diesel race boat after diesel raceboat run small dia props - my personal experiences aside how can they all be wrong?

I'm not disputing you, I want to know what experiences do you that have that make you say what you said?

Joe Gere
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Old 11-04-2008, 11:46 PM
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Most of the stuff we have played with have been in the 18-21 range.

Most recently we have been doing a few 50' boats with twin 480s running 21x32 Acme props for speeds in the 64-67 mph range. Granted we have been running ASD8's so we are not limited in diameter.

the 39 nortech that the boss has is running 18X32's I believe and that boat runs in the 80-83 range consistently.

The only Nortechs running the bravos and diesels are the 43's with triple 6lp 315s. those are limited on diameter as well.

All I am getting at is the majority of stuff we run, perform better when you can take advantage of the larger diameter.
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Old 11-05-2008, 08:00 AM
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I understand what your saying. I wasn't comparing diesel boats I was shooting for gas powered boats which I didn't make clear.

On the Nortech that does 83 what's the gearing?
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Old 11-05-2008, 01:11 PM
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I didn't ask about fuel rate to calculate to actual HP. If they haven't done it yet, they will.

Since its full-authority electronics, and the statement was made that it was running ~70 at rated speed (3400 rpm), this only suggests that they're not over propped with the 17x37.

If the governor begins to pull fuel right at 3400, allowing only 50 rpm of no-load overshoot, they could be under propped. I thought I heard that they tried the 17x39 first, so this would be a counter-point to my previous statement. If they couldn't pull the 39 to rated speed, I'd say they're pretty close to using everything the engine has in it.

This isn't to say that the efficiency of that particular combination is as good as it can be. That type of change seems like something that gets you 1-2 mph.

Purely from a top speed perspective, which is more efficient...bigger wheel, reduced pitch, same rpm or smaller wheel, reduced pitch, higher rpm?

There are an many ways to put 480hp into the water, and each has tradeoffs. I'm not so sure they haven't hit one of the better ones right out of the gate.
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Old 11-05-2008, 07:47 PM
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he is running a 1.18:1 ratio. I should also say that we really only play with the larger diameter stuff on the LY engines and arnesons. We don't do many arneson boats with the smaller LP engines.
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Old 11-05-2008, 08:11 PM
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what are you guys thoughts on multi speed trannies??
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Old 11-05-2008, 08:33 PM
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obrein,

So you validated what I keep saying which is "you need prop speed to go faster" not pitch or dia.

Your running the 1.18, just look some real loose logical deduction here.

1.3's put you in the 70's
1.15's put you in the 80's
.8's I know from the EBEL put you in the high 90's
.6's I know from Gancia put you in the mid 110's

You see the pattern .2 taller gearing gives you about 20mph in every case. I know this is loose but the pattern and the actual applications prove it to be a logical deduction.

There are many problems with tall gears so when stainless asks "what are you guys thoughts on multi speed trannies??".

That is how you overcome the pitfalls of tall gears. I wrote alot on here about our projects with Twin Disc and the yellow Don Shead boat from the UK with #3 drives and it's hydraulic "slipper" overdrive trans.

Stainless, the theory behind them is 100% sound - I have never had the luxury of using them as they did not exist in the form they are today. We ended our work almost 20 years ago.

Joe Gere
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Old 11-05-2008, 08:37 PM
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Thanks joe. The new cms boat is using some close ratio 4 speed units. It might be cool to try something along those lines with a diesel app.
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Old 11-06-2008, 11:17 PM
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habanajoe,

I agree completely on needing prop speed. However we have had luck on several boats running more diameter that gave us higher efficiency. The 50' we have been running has a 1.5 ratio and runs 67+ in the 3450-3500 range which is all we can turn rpm wise.

I quess what I have should have said earlier, is that they definitely need more ratio to run the size props they are trying right now (i.e. more prop speed). with the ratio they are running I was only suggesting that they try a bit more diameter rather than continually adding more pitch.
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Old 11-07-2008, 01:54 PM
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obrein,

I agree with you, I can tell your a smart guy.

I like to debate things and see what comes out of it.

I think we're all on the same page here, everyone wants to see this boat do great.

On a whole another note didn't Weisman make multi-speeds for a couple Seatek race boats? That might be what this boat needs a 2 or 3 speed with OD to raise the 1.32 up toward direct?

I know a little late in the game now, but after the last two days I'm not getting any new boat for awhile so I have to live through everyone else for now!!!!

Joe Gere
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