Reply With QuoteThat just got technical like... http://free.***boat.net/ubb/biggrin.gif
RD
Mr. Havasu Cat, I hope you don't mind me trying to help make your DCB run better slip numbers. I have been giving your stats a lot of thought. I have compared it to the data from my boat and also to the research I have done on other Mach 26's tested in the mags.
This is what I believe your facts are. You ran 105mph @ 5800rpm with a 1.50gear and 32pitch prop. Good performance, but sloppy out of the hole. Boat was loaded with fuel and had passangers. By my calc, this gives you a slip of 10%. Then to improve the out of the hole performance, you tried a 1.35 gear. This netted 104mph @ 5800rpm with 1.35gear and 30pitch prop. Boat was loaded with fuel and had passangers. You didn't say if this improved out of the hole performance. Again, by my calc, this gives you a slip of 15%.
Here are my comments and observations. First, going from a 1.50 to a 1.35 is the same as increasing your prop pitch by 3 sizes. Therefore, a 1.50 with a 32 pitch is roughly the same as a 1.35 with a 29 pitch. You ended up with a 1.35 with a 30 pitch, making this combo taller than your previous 1.50 setup. You went to a taller setup yet you were able to pull the same 5800rpm. Where did you get the extra HP to pull the taller gearing to 5800? My guess is you may have had the boat trimmed out too far, making the slip % go up and allowing your motor to turn the taller setup to 5800rpm. In other words, your motor didn't make any more power, but the trim setting allowed in to pull the taller gear to 5800rpm.
Why do you have more slip with the taller gear. The answer is you should not. According to Fred Inman of Imco, a 28pitch prop is the ideal size and generates the least amount of slip. You are closer to this ideal prop size with your current setup than you were with your 1.50/32 setup. If this statement by Fred is true, you should produce less slip with your 1.35/30 setup.
You should be able to repeat the 10% slip pass you made with your 1.50/32 setup. I calculate a top speed of 110mph @ 5800rpm with a 1.35/30 setup using 10% slip.
February 1999 Hot Boat magazine. In this issue (I think you can get a copy of this article from this web site) they test a Mach 26 with a blown 509. This boat ran 111mph at 5600rpm with a 1.36 gear and a 30 inch prop cupped to 32 inches and lab finished. For my calculation of this slip, I assumed a 31pitch because I didn't know how to treat the prop mods. This eguals a slip of 8%. (I JUST GOT AN IDEA, WOULDN'T IT BE GREAT IF HOT BOAT WOULD PROVIDE THE SLIP % ON THE BOATS THEY TEST)
To get the low slip % you need the right water conditions and load or lack of load in the boat. If not, you may be trimming the boat to compensate for these factors, putting the drive in a less that ideal position for top speed. Get the ice chest, anchor and passangers out.
Here is my prediction, First, Your X dimention is in the right spot. At Havasu, with the right air and water conditions, the correct trim setting, a very light load in your boat and your current 1.35/30 setup, you will run at 8% slip netting a top speed of 114mph at 5800rpm.
Like I said in another posting, I am happy to compare my info to that of yours.
SVO 540 - Out.
That just got technical like... http://free.***boat.net/ubb/biggrin.gif
RD
SVO 540: I have a question for you regarding Prop sizing: I just took delivery of a 26 ft cat. I have 502 with a vortech supercharger putting out approximately 575HP. I recently ran the boat and achecived the following 74.5 MPH on the GPS, I had the drive trimmed all the way down and was hitting the revlimiter at 5300. I was running a 28 ptch Bravo four blade. I know she has plenty more to give but somwhere the numbers just don't add up. I'm confident the speed ws accurate because of the GPS. Can you please make a prop suggestion for my next test and let me know what you think my numbers will reflect based upon your sugesstion? Thanks in advance
damm gimprivr,something dont right
SVO540,
I definitely think you are right regarding the slip numbers. I think the boat with less weight and better conditions would run faster, and yes the boat is much better out of the hole now with the new gears and smaller prop. I think the cooler air temp this past weekend helped with a little more hp and let me pull the 30" prop to 5800.
I actually know the guy who owns the boat you are talking about from the ***boat test. That boat is running a standard (non xr) drive. I think the xr takes more hp to turn than the standard bravo from people I have talked to that have switched drives and lost speed. I am going to try a 28 inch prop this summer in the heat and see if I see better numbers. The 30 in the heat of the summer is just to much prop unless I decide to add some boost. Thanks for the help.
Originally posted by MrHavasuCat:
SVO540,
I definitely think you are right regarding the slip numbers. I think the boat with less weight and better conditions would run faster, and yes the boat is much better out of the hole now with the new gears and smaller prop. I think the cooler air temp this past weekend helped with a little more hp and let me pull the 30" prop to 5800.
I actually know the guy who owns the boat you are talking about from the ***boat test. That boat is running a standard (non xr) drive. I think the xr takes more hp to turn than the standard bravo from people I have talked to that have switched drives and lost speed. I am going to try a 28 inch prop this summer in the heat and see if I see better numbers. The 30 in the heat of the summer is just to much prop unless I decide to add some boost. Thanks for the help.
MR HC your choice of a 28 prop is good but you might want to try a non lab it hooks up better out of the hole, the xr takes about 10hp more than standard drive but i think a good 28lab/10 lbs boost maybe1/2 tanks of gas and it will work just fine
Originally posted by gimprvr:
SVO 540: I have a question for you regarding Prop sizing: I just took delivery of a 26 ft cat. I have 502 with a vortech supercharger putting out approximately 575HP. I recently ran the boat and achecived the following 74.5 MPH on the GPS, I had the drive trimmed all the way down and was hitting the revlimiter at 5300. I was running a 28 ptch Bravo four blade. I know she has plenty more to give but somwhere the numbers just don't add up. I'm confident the speed ws accurate because of the GPS. Can you please make a prop suggestion for my next test and let me know what you think my numbers will reflect based upon your sugesstion? Thanks in advance
No problem. What is your drive gear ratio. I would guess 1.50 based on the info but it is better to confirm. Also, what max RPM to you want to run a wide open throttle. You need to stay off the rev limiter to get max speed. With that, I can guess at it for ya.
SVO450
Man you must have cleard out all the beers in the brain, way to much thinking goin on.
oh by the way left message on your work voice mail, Trent is to funny!!!!!!!!
Mr Havasu Cat, I found in the testing that I have done that a non-lab prop has less slip through the mid range and has more slip on the top end when compared to a lab prop. This is consistant with what mrpumpshtm said. For family boating at Havasu, I prefer to run a non-lab 28, but for the big top end numbers, I run a lab 30.
The info you gave me still doesn't add up. If you assume that the additional RPM dicussed in the opening post is due to better air, then why did your slip go from 10% to 15% with your prop and gear change? If the drive trim was the same in both passes, the only answer is additional weight in the boat.
I have found that my mach finds an overdrive gear and really gets aired out when there is no extra weight in the boat. Also, when I am running light and really flying, my trim is between 0 and +1. Add more weight and I have found that I need to add more positive trim and muscle the boat. It doesn't get that overdrive gear sensation when there is a full load in the boat.
Make your speed runs with a light load and you may find this overdrive gear sensation. I would be happy to take you for a ride in my boat so you can get a feel of what I am talking about. Your hull is the same as mine so I am sure it's there.
I hope you don't mind my comments, but I have one more. I am concerned that you will turn to many RPM with the 28pitch prop. You are currently turning your 30pitch 5800rpm. Remove two cup sizes and you might pick up 300 to 400rpm. If so, this puts your rpm at 6100 to 6200. If you have a solid roller cam, this is ok, but my guess is it is hyd. Talk to your engine builder about max rpm and desired rpm at full throttle.
As you can tell, I love this shit and would be happy to help with your testing and also let you try my props, etc.
About gimprvr's post-you say you had the drive all the way down. That's a rpm/mph killer to me. Was there any up trim performed? Should be in the 85-90 range.
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