HPJunkie

My boat porpuses at 93 mph and I think that the drive could use more leverage to hold the bow up and increase speed. I hate to buy one and find out that it wasnt the problem but the way it feels it needs more bow lift. I was thinking about starting with a 1" and then a 2 inch one. Should I go that way or 2 inch first? Who Makes them? Greg

zt260

Start with 1", but what prop are you running? rpm, ratio, and speed at WOT can tell us a lot.

HPJunkie

Well ZT, I have a Throttle up performance 4 blade bravo 1, 1.5 gear, 6200 rpm 93.5-94 mph. and its bouncing. Ill post a link or 2 so you can judge for yourself, then tell me what you think. Greg

zt260

Assuming it's a 28" bravo 1, I would not put money into a spacer unless you have one lying around. I would look for a Mirage 29" to try, it will lift the bow dramatically. If it's a 30" bravo 1 then a 1" spacer or 5 blade would be my next step.

How was this prop labbed and did you run it box stock first? What hull and drive are you running?

HPJunkie

ZT , the hull is an Eliminator 250 closed bow, It acted the same before the prop tune up, I did tell Julie what was wrong and they extended the cup and balanced it and blueprinted it as well. They did not thin it because of the shallow waters that I boat in. I have a brand new Mirage plus 29 and it wont plane off without blowing out. I have a new bravo 1 30" prop also that I dont use much because the acceleration isnt as great. I cruise at 28-3200 rpms now with the 28. The motor wont hold the boat on plane below 22-2300 rpms ( I use the 28 to ski also). I bought the 29 because Ive heard that the 3 blades will run 2-5 mph faster. I hate to blue print the 29 when a spacer might solve some problems. any suggestions would be great. What happend when you did mmy slip #'s. Greg

zt260

Well that changes a few things. Cats don't normally need more bow lift as they are naturally lifting. Do you have tabs? If so what difference do they make for you?

I would be looking in the opposite direction to solve this. A 2" shorty with a 5 Blade 28" maximus would be my next step. I would expect a top speed increase and better drivability. If you don’t want to go for the shorty now, I would try the 5 blade with your current setup.

HPJunkie

Well zt I dont know if your clear on this this particular boat....it is a V bottom not a cat. I have no tabs. trinimming in slows the boat down, hence more bow lift.
Check these links:
Shameless heres a couple links:


http://www.fullmooninjune.info/images/MVI_2608.AVI


http://www.fullmooninjune.info/images/MVI_2609.AVI

Thats the optimum trim setting. I think tabs would slow the boat. Greg

zt260

Ok, I forgot eliminator is making Vhulls. My bad, so ignore that last post.
A spacer might help the bounce, but it will slow you down so I would not look at that option. I would try to make the mirage 29 work. Seems odd you can't get on plane. Any chance you could try a 27" mirage plus? Tabs help in lots of areas, planning and controlling porposing. I run 380 K-tabs. Without them I would have a much harder time.

BTW how many inches below the hull is your propshaft?

I would call Brett and Bblades.com
He knows merc props inside and out. He just labbed my maximus and my blowout is all but gone.

throttleup

Your slip is at 12%. Which is good for your set up. Your bow does seem to be low. What happens if you put more positive drive trim in at WOT?

Does the boat porpoise at cruise speed? Does the boat porpoise in choppy water?

A 1" spacer will help to bring the bow up. I wouldn't go with more than 1", based on your numbers.

The spacer will not necessarily reduce your top speed, this is a common misunderstanding in the marine industry. There is an optimum drive height which will change based on the boat, power, prop, drive, etc. Finding that drive height is the key and testing with different drive heights is the only way to find out for sure. We have seen speed increases of 5-8 mph by installing spacers from 1-3" on boats that had the drives installed too high from the factory. Higher is not necessarily faster!

Matt

zt260

At 12% slip, the odds of a spacer helping your porpoising and not hurting speed are rather low. I would try working that Mirage over and start looking for tabs. My hull is very similar to yours. When I was running 800HP the mirage was best for top speed, but the bravo was better all around. Main difference is I had good tabs to control it.

Now I'm running a shorty Bmax and a maximus with a lot more HP. This could also work for you, but you will need tabs for sure.

The mirage plus can be labbed for less blowout, and the Bravo can be labbed for more bow lift. It just has to be done correctly. Good luck