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I can't figure out this math, please help.

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Old 08-24-2008, 03:50 AM
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The more reduction gear you have the slower the boat will go. You need to go the opposite direction like 1.33, 1.25 not 1.63.
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I think this way too. I think the 1.50:1 is too tall for this set-up.
To get speed he has to go with gears shorter than 1.5 BUT!!! The Yanmar 6LP is comparable to a CMD QSD 4.2. The shortest gears you can get in Bravos with that engine are Bravo-1 1.36, Bravo-2 1.65, Bravo-3 1.36. Those a out of the box Mercruiser WARRANTED setups. By the time ole boy gets the gears that will give him the speed they won't stand up to the torque.

Joe long time no see.... Ed
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Old 08-24-2008, 05:37 AM
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Assuming the motors will pull the prop a 30 pitch at 3800 at 10% slip will put you at 64 mph,at 32 pitch at 15% slip will put you at 65 mph,you can play with the numbers on the prop calculator website,Smitty
http://www.rbbi.com/folders/prop/propcalc.htm
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Old 08-24-2008, 11:21 AM
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articfreinds,

I'm always argumentive so please this is not against what you found with the prop calculator.

Here is the problem I found with those prop/boat speed calculators (and I'm not smart enough to allow or this). They do not take into consideration the efficiency/thrust of the prop, I know you plug in "slip" but I mean at what rpm does the prop start to work right or push enough water not distance traveled as in pitch.

We all know until the prop starts spinning at a certain rpm it doesn't not push enough water to overcome the drag of the boat well. If you changed to a larger dia prop it would overcome drag at a lower speed i.e. the larger the dia the slower the speed you can stay on plane (providing you have the Hp).

What I have seen most boats don't start really running well until the last 20% of their rpm range, least slip etc. If I used that as a base and let's say the gas engines that spun Metal Bros props on the Fountain before turned 5,000 rpm they would have a prop speed of 3,333 rpm. If I used my 20% rule then at 4,000 rpms the boat is still running well which is 2,666 rpm of prop speed.

What I'm saying in the gas boat example is the prop needs to spin at least 2,666 rpm's to work well.

His Yanmar's will turn 3,800 rpm's max with that same gear he has a 2,533 rpm prop speed at max and a t cruise of say of 3,200 rpm his props are turning only 2,133 rpm's.

So, I think based on nothing but speculation (here is where if he had some data from the existing boat he could make assumptions and get to a closer answer) and some practical experience that his slip will be very, very high and the boat will not come close to what the prop calculator says for the shear reason the props on those drives will be too small to make enough thrust (not dstance but pounds) to push that boat at 2,500 of prop speed.

After all that above I still say and it's been proven time and time again Hp is Hp - a diesel is not a magic bullet. 10hp of diesel is exactly the same as 10 hp of gasoline. If I hook both 10 Hp engines up to a gear box that will turn a pulley at 500 rpm's then the torque of both engines at the pulley will be exactly the same regardless of engine rpm's - everything is equal!

Metal bros - how fast would the Fountain run today with 315 hp of gasoline engine?

Joe Gere
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Old 08-24-2008, 12:01 PM
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I love threads like this. I know what you mean just because of all the work I did with props on Gas RC boats. Same pitch on a 2" diameter prop vs a 3". Will not even move a gas boat like a nitro. We had to modify/make our own. What about BBlades and ThrottleUp? The other one to question may be BuilderBob? Aren't his Nortechs almost identical except for Diesel vs Gas?
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Old 08-24-2008, 12:26 PM
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That little 315 Yanmar is a great engine but regardless what they say about it maxing at 3600-3800 I would prop it for 4000 or better.I had a problem with one creating a lot of heat in the original suggested operating range of 3600-3800.Insiders said prop it for 4000-4200. I did and that was all it took.That 315 is sensitive to Lugging

I had one that I put 4000 hours on.Had a lot of problems because of the heat factor from overpropping.The engine is a mairinized Toyota Diesel that I think was manufactured for excavating equipment.
Fuel burn is 9 gallons an hour at 3200 which was my minium planing speed and 15 gallons an hour at 4000.

Last edited by tommymonza; 08-24-2008 at 12:38 PM.
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Old 08-24-2008, 12:39 PM
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I thought the Nortech's were Arneson drives? I would bet they are closer to direct drive. You all look at your own boats, you have some fast boats, they all have one thing in common higher prop speed.

I tried to sandbag one of the Miss Geico guys on here one time with questions leading up to prop speed, he caught on what I was driving at but I got my answer which was way up into the 4,000 rpm. Again, can't go fast by spining a BIG wheel slow.

Like my freind Firefox says, you gear up the drive the torque will break it, which is why you do overdive inside the boat before the drive.

Joe Gere

Last edited by HabanaJoe; 08-24-2008 at 12:44 PM.
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Old 08-24-2008, 03:48 PM
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Originally Posted by tommymonza View Post
That little 315 Yanmar is a great engine but regardless what they say about it maxing at 3600-3800 I would prop it for 4000 or better.I had a problem with one creating a lot of heat in the original suggested operating range of 3600-3800.Insiders said prop it for 4000-4200. I did and that was all it took.That 315 is sensitive to Lugging

I had one that I put 4000 hours on.Had a lot of problems because of the heat factor from overpropping.The engine is a mairinized Toyota Diesel that I think was manufactured for excavating equipment.
Fuel burn is 9 gallons an hour at 3200 which was my minium planing speed and 15 gallons an hour at 4000.
Was that one out of a pair that was creating heat? What did your pyrometer read when you were running wot???
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Old 08-24-2008, 03:54 PM
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Seems to me this would be a great application for Konrad's bravo replacement drives??? Sell the XRs and move on up.
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Old 08-24-2008, 04:06 PM
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Originally Posted by HabanaJoe View Post
articfreinds,

I'm always argumentive so please this is not against what you found with the prop calculator.

Here is the problem I found with those prop/boat speed calculators (and I'm not smart enough to allow or this). They do not take into consideration the efficiency/thrust of the prop, I know you plug in "slip" but I mean at what rpm does the prop start to work right or push enough water not distance traveled as in pitch.

We all know until the prop starts spinning at a certain rpm it doesn't not push enough water to overcome the drag of the boat well. If you changed to a larger dia prop it would overcome drag at a lower speed i.e. the larger the dia the slower the speed you can stay on plane (providing you have the Hp).

What I have seen most boats don't start really running well until the last 20% of their rpm range, least slip etc. If I used that as a base and let's say the gas engines that spun Metal Bros props on the Fountain before turned 5,000 rpm they would have a prop speed of 3,333 rpm. If I used my 20% rule then at 4,000 rpms the boat is still running well which is 2,666 rpm of prop speed.

What I'm saying in the gas boat example is the prop needs to spin at least 2,666 rpm's to work well.

His Yanmar's will turn 3,800 rpm's max with that same gear he has a 2,533 rpm prop speed at max and a t cruise of say of 3,200 rpm his props are turning only 2,133 rpm's.

So, I think based on nothing but speculation (here is where if he had some data from the existing boat he could make assumptions and get to a closer answer) and some practical experience that his slip will be very, very high and the boat will not come close to what the prop calculator says for the shear reason the props on those drives will be too small to make enough thrust (not dstance but pounds) to push that boat at 2,500 of prop speed.

After all that above I still say and it's been proven time and time again Hp is Hp - a diesel is not a magic bullet. 10hp of diesel is exactly the same as 10 hp of gasoline. If I hook both 10 Hp engines up to a gear box that will turn a pulley at 500 rpm's then the torque of both engines at the pulley will be exactly the same regardless of engine rpm's - everything is equal!

Metal bros - how fast would the Fountain run today with 315 hp of gasoline engine?

Joe Gere
Right @ 60-mph would be my guess. I have already put in an email to to Max Machine Worx guys, I've never met them but their work is well spoken of in our area. I will have them do a 1.36 swap and if they have any goodies they can add while they are in there we will do that too. None of this happens til Feb anyhow so there is time to get a solid plan together.
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Old 08-24-2008, 04:07 PM
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Quote:
Originally Posted by fantastixvoyage View Post
Seems to me this would be a great application for Konrad's bravo replacement drives??? Sell the XRs and move on up.
I would love to, not in the budget though.
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