Hey guys. I bought a 23.5 larson senza last year with 21p 3blade prop. The boat goes pretty nice out the hole but my speed is just horrible. At 4.500rpm the boat is only moving 30mph. I think the boat should be able to do better than that. I was thinking of going with the 4 blade 23 pitch? Any help will be highly appreciated.
Nick

usually with 4 blades you loose a lit top speed but pick up cruise speed.. Best thing to do is find a buddy with a 23 p 3 blade, mirage plus etc and give it a try., Is your current prop chewey? because something is not right. At 4500 rpm with a 21 you should be over 10 mph faster. Check the drive ratio too, If the drive was swapped it could be off.

What ratio drive? Something is not right by a bunch. Boat should run low to mid 50s with that prop and a 1.50 ratio drive. As long as prop is in good shape it's right in the ballpark of what needs to be on there.

Just as an example i have a 454 alpha and a 23 pitch prop. I have a 1.32 alpha and run 45 at 3000.

Thanks for the replies guys. The drive is a 1.5 from sei. The prop is in good shape. The motor has low hours and is a 98 model 454 330hp. Maybe the overall size of this prop is off? Could that throw eerything off by much, previous owner was not into boats much so he has cheaped out on many things on this boat hence the reason i had to replace the motor and new drive. I will try and see te exact measures of the prop after work today.

Are you trimming up fully ??? What make of prop??? Is that a GPS speed???

Yes im trimming good. I am not sure of the make. I will have to take some pictures and post it up. Yes used a gps.

think so,Just as an example i have a 454 alpha and a 23 pitch prop. I have a 1.32 alpha and run 45 at 3000. thanks

Alpha One back in that day Merc did have 454 engines matched up to them with a gear ratio of 1:32. In 1988 when the Bravo was introduced no more alpha's were used behind big blocks coming from Merc. I would stay with a 3 blade as it usually is a little bit easier on the drive compared to a 4 or 5 blade prop.

Your speed is way off or your speedometer is way off or the hub in your prop is slipping. Is this a recon prop - maybe pitch way off or bad rebuilt prop ?

Also is your boat carrying a lot of extra weight as another possibility - maybe water leaking in and or saturated foam / stringers - just saying as possible reasons why your speed is way off. Bottom of the hull very clean and in good shape also no anti fouling paint ?

IMO you need to try a new 23 pitch prop and possible check your RPM's to verify the tach is working correctly. Your boat should be a 50 mph at least. Your original drive ratio should have been 1:32 with an alpha matched to a 454 back in the 80's. I know SEI does not offer 1:32 gear ratio in any of the Alpha's..

How do you have a 1998 Motor matched to an alpha anyways ? Your Larson must be an 80's boat with a more current repower, is that correct ?

Thanks for the replies guys. The pitch on the boat was 20, i just pulled it off today. The boat is basically empty hull other than a few ropes, anchor and usually small sized cooler full of drinks. When i bought the boat it had low pressure, i was told the motor was fully rebuilt, the previous year but spun a bearing. I rushed way to fast into this boat. Brought it home took motor apart and as soon as i was about to drain oil and i noticed more water came out than actual oil. As i started digging in more i decided to scrap it and buy a newer 454, if i remember correctly the previous owner told me it came out of a 98 four winns or formula im not sure. Anyway the motor runs amazing starts everytime. The first season running this boat which was last year i blew two drives than decided to just purchase a sei. Lowest gear ratio was the 1.49 they had. I just purchased a 3 blade 23 pitch laser2 prop. Hopefully the weather holds up and i get s chance to test it on the weekend.
Nick