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| rpm's and top speed' https://www.mudmotortalk.com/mmt_v2/viewtopic.php?f=3&t=2873 |
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| Author: | JD [ Mon Jun 29, 2009 6:04 pm ] |
| Post subject: | rpm's and top speed' |
My rig seems to top out speed wise at 4500 rpm. It will run 4850 with out popping up but I gain no more speed. I can bury the prop and still turn 4350 rpm and run with in a mph of top speed. I am running a 20x60 800lb rig with a full blown stage 3 35 - hp gdsd. I am thinking prop slippage is a problem because I am pushing such a heavy load. If I drop my motor a 1/2" to 3/4" its like adding a 1" to 11/2" of prop Dia. Trimming down would only change the angle of attack. I need to get a little more prop in the water. Have any of you guys fine tuned transom hight for max performance. What did you find? JD |
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| Author: | craig [ Mon Jun 29, 2009 6:31 pm ] |
| Post subject: | Re: rpm's and top speed' |
I think most people fine tune with prop. What kind of speeds are you getting with that big boat jd? What size prop are you running? |
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| Author: | JD [ Mon Jun 29, 2009 7:10 pm ] |
| Post subject: | Re: rpm's and top speed' |
I am running the gd 3 blade it has about 1/8" of wear. With the evt internals I was touching on 30 when the cam failed. I had to go with bps parts on rebuild, 20 over on pistons and BPS rods and cam. I had to keep the evt rollers. I was playing with jetting and found a tear in my carb mount but was still touching on 29.5. I am running two less teeth on the top gear and turning a little more rpm for the same speed. The boat flys out the hole and climbs real fast. My buddy has a 16 X60 gd and a gdsd with stage 1 kit ,evt cam and head work. It runs 32 mph and is fast out the hole. I pretty much stay with him out the hole and he walks away slowly after I top out. It moves for a big boat. I have played with modding longtail to run surfaced , I cut the shafts,and lower the motors by setting them in a well. When I got it right they would double the speed of a long tail. Having the right amount of prop in the water and right angle of attack made a big difference. I have a pad on the bottom of my rig, if lowering the prop will get the rig riding higher and faster It may run just on the pad, if it does it will really fly. JD |
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| Author: | Gigafowl [ Mon Jun 29, 2009 7:47 pm ] |
| Post subject: | Re: rpm's and top speed' |
JD, Sounds like you changed your gear ratio from 1.4 to 1.6 or 1.7 to 1... Prop wear - you say 1/8" lost - how are you measuring her ??? GD 3 blade brand new is about 5 & 9/16" from center to blade tip AND more importantly 3 & 1/2" across the fat part of the blade... I am testing a demo stock GDSD 35hp with 15 hrs on the demo prop and it measures 5 1/2" from center to blade tip AND 3 1/4" across the blade... That prop is 3 mph slower than my other used GD 3 blade which has about the same measurement from center to tip BUT is 3 7/16" across the fat part of the blade - only a few 1/16s difference equals 3 mph... My rig is the GDSD 16x48 @ 27 mph on demo prop and 30 plus on my other less used 3 blade... My buddy ran his 3 blade to a nub - almost no loss from center to tip of blade but at 3" across the fat part of the blade it would just churn and only barely plane out the hull with 2 guys on his GDSD 18x48... SUGGESTION - if you have a good prop consider going back up on ratio - or - try the older GD 12x12 2 blade -- it tests even faster on my hull than the 3 blade which I think I heard was a 11x11... When buddy has his rebuilt he's gonna add a few more mm of cup to the trailing edge - I will be interested to see what that turns out like... |
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| Author: | JD [ Mon Jun 29, 2009 8:12 pm ] |
| Post subject: | Re: rpm's and top speed' |
I am 3 1/4 wide at the fattest part of the blade. The prop has 25 or so hours on it. I don't know what the blade length is. I have a hard blind that goes on the boat that adds 150 lbs or so. The higher gears give me more power, I will set up one prop for light loads and top speed(More pitch and cup)and use a standard 3 blade with extra cup for my hunting load . It should be easier to change props than gears. You are right about prop wear and performance. If a 1/4" makes a 3 mph difference ,think about being a 1 high and attacking the water at a wrong angle will do (less surface are to push the load,to steep of a angle for best speed). Out board go faster than inboards for the same hp because the prop attacks the water straight on. We won't ever have that with our surface drives , but we can set things up to make the best use of what we have. JD JD |
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| Author: | craig [ Mon Jun 29, 2009 8:14 pm ] |
| Post subject: | Re: rpm's and top speed' |
Mud and sand will eat a prop up in no time! |
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| Author: | JD [ Mon Jun 29, 2009 8:26 pm ] |
| Post subject: | Re: rpm's and top speed' |
Since they started the diversion reggio is real hard on props. Watching my Buddy's go devil rig run, his boat was on perfect plane and the motor was parallel to the water. You could see that the angles GD designed into the SD were at work. It appears that my rig attacks at a steeper angle than his rig. JD |
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| Author: | Gigafowl [ Mon Jun 29, 2009 8:37 pm ] |
| Post subject: | Re: rpm's and top speed' |
JD Did you say you modified yer skeg ? With the length of the GDSD and the dynanic / hydrolic prop control IMO - transom height is critical but transom angle is a NON-factor... My Weldbilt is pushed by a 27 GDSD and it's transom angle is so wrong that I can't get the prop to clear the water.... But that rig hauls AZZ at about 29mph with the stocker 27hp and 12x12 2 blade.... But at 350lbs the hull is superlight... |
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| Author: | JD [ Mon Jun 29, 2009 9:04 pm ] |
| Post subject: | Re: rpm's and top speed' new data to report |
You just said it better than I could. With the length of the GDSD and the dynanic / hydrolic prop control IMO - transom height is critical but transom angle is a NON-factor... Transom height is what determines the attack angle of the prop. Too high of a transom you attack a little steep and do not have enough blade wet, to low of a transom you attack too shallow and have to much of blade wet at proper angle. JD Well I was getting 29.5 as post before. I cut 3/4" off the transom and added a 1/4" plate for a net reducion od 1/2" Two things became clear. One it was holding the bow up a lot better, two it was running 29.9 and 30.3 strong.That me at 260 and todd at 225 and 1/2 tank of gas 7 gals. Todd drove it and notice my handle was floating , he suggested that I bend the Tabb's up to make rest against the trim. That makes sense every time I hit 30+ it starts floating real bad. i made one run by my self down wind with a light ripple in deep water , i strong armed the handle to stop it from floating, "drum roll please" 31.6 mph for a 1/4 mile or so.The arm gave out back to 30mph. I am spinning 4550 to 4800 . Max speed is at 4550-4600. I got a new prop coming and will report back after the install. Jetting is not quite right yet,but getting close. JD |
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| Author: | JD [ Thu Jul 02, 2009 8:23 pm ] |
| Post subject: | Re: rpm's and top speed' new speed to report |
Well I was getting 29.5 as post before. I cut 3/4" off the transom and added a 1/4" plate for a net reducion od 1/2" Two things became clear. One it was holding the bow up a lot better, two it was running 29.9 and 30.3 strong.That me at 260 and Todd at 225 and 1/2 tank of gas 7 gals. Todd drove it and notice my handle was floating , he suggested that I bend the Tabb's up to make rest against the trim. That makes sense every time I hit 30+ it starts floating real bad. i made one run by my self down wind with a light ripple in deep water , i strong armed the handle to stop it from floating, "drum roll please" 31.6 mph for a 1/4 mile or so.The arm gave out back to 30mph. I am spinning 4550 to 4800 . Max speed is at 4550-4600. I got a new prop coming and will report back after the install. Jetting is not quite right yet,but getting close. Fastest speed is a little before WOT. High speed jet is not quite right. Mid range is real strong and it jumps on plane as fast as a 16x60 gd hull , or a 16 foot GTR with gtr running stage 1 with evt cam, that rig tops out at 35 + The gd rig tops out at 32 +(It is a larger heavier boat) JD |
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| Author: | Gigafowl [ Thu Jul 02, 2009 8:30 pm ] |
| Post subject: | Re: rpm's and top speed' |
Bet you will be at 35 mph's door with the new prop... The prop lift will go away too at WOT - symptom of worn prop... Post back with numbers on the new prop... |
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| Author: | JD [ Thu Jul 02, 2009 8:37 pm ] |
| Post subject: | Re: rpm's and top speed' |
| Author: | RC [ Fri Jul 03, 2009 12:37 pm ] |
| Post subject: | Re: rpm's and top speed' |
When nickel and diming for speed, True transom height is a big factor with top end speed and overall performance. The transom angle only affects true transom height, doesn't affect anything other than that. Had my last hull built with a "true" transom height of 19.5" (measured vertically from ground, with boat sitting on the ground). This put the gear knuckle just barely out of the water, on a PD, giving it the flattest angle possible and extended downward digging ability as a bonus. |
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| Author: | JD [ Fri Jul 03, 2009 9:06 pm ] |
| Post subject: | Re: rpm's and top speed' |
first I have a retraction to make! My buddy Todd only weighs 215. Coco is such a great guy, I called yesterday,and a new prop and new prop wrench showed up on my door step to day. I put the new prop on and a fuel air ratio Gage in today . The new 3 blade prop was 3 1/4 wide across the blade. I only had a 1/8 wear on the old blade. The new blade performed like the old one -same rpm and speed. 28.5 -30.3 I did not see 31 today.Todd called it right yesterday when he said conditions were perfect for max speed. ( a good breeze with a soft ripple on the water.) Since I cut the transom the rig runs much better in a chop. I am able to keep the bow up now. It ran too flat before.If I trim out past 4550 I gain little or no speed, I can trim down to 4300 and still do 27.5. I bent my tabs up to make the motor trim better and make it hold speed and rpm steady instead of floating. I am getting some spray back off the left side of the shaft. I don't know where it is coming from. It's got to be causing drag. The fuel air gage worked all of one minute and not pass idle.so it was of no help. I lose speed the last 1/8 of throttle. I think I an too rich. JD |
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