So.. Is 75 mph gonna be it????
 

I have a 21 boot baja islander witha 502 that dynos at 520 hp at 5100 and 530 lbs tor at 4500. I just installed gils and went back to prop testing. Ive tried 23 and 25 lazers and mirages and 22 b1 and 23 trophy. All of these pull into the rpm range I need and the best has been 71 with the 23 trophy 4 blade and 74.1 with the 25 lazer. In 90 deg heat and at 2000 feet alt. I was hoping for more , but maybe thats the wall. TILL I find the dough for the roller valve train!!!

Depending on the year of your boat, I think it has the same hull as mine (97 hammer). When deciding what motor to buy with it, I contacted the factory as to relative speeds with different engine packages. 300hp 7.4 62-66 (70 speedo). 415hp 8.2 72-76 (80 speedo). I'm sure these are at sea level and are very close to numbers that I saw after buying the boat. Where I'm going is that if you really have 520hp (dyno?) then you have around 35 less at the prop (485). That's only 70hp more than the 502 equiped version which did 72-76 at sea level. Was the dyno info collected at the same elevation? I don't know the conversion off hand, but have heard others mention it and I was suprised at the loss with even a little altitude. It seems that 70hp might easily go away with 2000 feet which would put you right in the ballpark of what that hull has done in the past. There are several others here that could estimate the hp loss at 2000 feet better than I. If you are always going to boat at that altitude, you might consider a boost in compression next time around.

Good luck!

I forgot to mention,

I've tried laser IIs in the past and they slipped like mad on my boat. I'm sure we have similar weight etc. The Mirage and + was the ticket for my boat. Do you have any numbers using Mirage 3 blades? I would think a 23" would be about right. You might be spinning into the mid 5000s, but that should be okay.

Hey Steve>>As you know my setup is about the same as yours. When we installed the 502 I could pin the spedo at 80 but that's the spedo??? I didn't try it on GPS. Are your numbers spedo or GPS?? If they are GPS it's probably pretty close.

syringa

2000 feet and 90 degrees will cost you ~9.5% vs. 90 degrees at sea level and 77 degrees.
520*.095=49.4 hp loss.
I feel your pain! We live at 5000 ft.
:mad:

Hi guys... syringa mine speeds are gps , hows your boat doing? I havent tried a 25 mirage yet , ill see if I can find one. When you read boat tests like the last hotboat where a 25 ' cole went 75 with a stk 502 you wonder if you are doing something wrong.
My engine was built by john lingenfelter, when it was dyno'd the #s were corrected for see level and Im sure the engine was run using headers [ i dont imagine they would bother with the hassles with marine manifolds] They put the stk manifolds back on and thats the way I bought the boat.I have just installed gil exhaust now . So Im guessing the power to be in the low 500s [oh I have changed to a MSD ign as well] . Its tuff to tell what each hull wil do i guess. I was shooting for close to 80. sj

Hey Steve>> Sounds like your boat is running fine. On my milkshake nightmare >>it might be over. Got the engine back from dynoing yesterday and installed it today. I switched back to the original cooling system with the circ pump and t stat. I want to keep the temp up and the water pressure down. I'm going to run it tomorrow and we'll see.

syringa

Syringa.. what was the problem? Youll have to get a gps some day and let me know what you are getting for mph. We seem to have a pretty close setup. Have you ever found a 4 blade that got you close to the speeds of a 3? So far everyone Ive tried lose at least 4 mph to the 3 blades. The over all performance seems best with a bravo1. What rpm are you maxing at?
thanks sj