Any ideas on how to pick up speed?

[solar7647]

I have a 3.7L 224cid Mercruiser with a alpha one drive. The boat is 22ft and about 3600lbs. I am running at 4600 RPM at WOT, with a 14.5x20 4 blade prop. The engine is running great holding proper oil pressure, temp and holding RPM steady. I can come up another 200 RPM still and be with in proper range but would like some input (any input) on how to pick up some speed. I read several places that if the transducer for the depth find is to close to the drive in will increase cavatation, do you think moving out to the side would help. I am currently topping at 33-34mph according to GPS. Thanks

 

[CheapboatKev]

Re: Any ideas on how to pick up speed?

What about trim tabs?

 

[redone4x4]

Re: Any ideas on how to pick up speed?

isnt 33-34 mph about all that engine will do in a 21ft hull? that seems pretty acceptable to me. heck, i dont even know if my 19 with a 305 in it does over 40.

 

[Boss Hawg]

Re: Any ideas on how to pick up speed?

Why in such a hurry
Everyone seems to want to go faster these days

 

[marine4003]

Re: Any ideas on how to pick up speed?

isnt 33-34 mph about all that engine will do in a 21ft hull? that seems pretty acceptable to me. heck, i dont even know if my 19 with a 305 in it does over 40.

Yea, really. Your not gonna get anymore out of it,as stated above increasing hp ,even by 50 hp doesn't do much,all your gonna do is throw money into ?go-faster"options,and you'll be lucky to pick up 1-2 more mph. In racing boats,for every $1000 you spend all you get is 10 mph.....

 

[dingbat]

Re: Any ideas on how to pick up speed?

You can start with a new prop. If your numbers are correct your theoretical speed is 48 MPH. With a top end of only 33-34 mph your looking at 31% prop slip. Should be a lot less slip

 

[redone4x4]

Re: Any ideas on how to pick up speed?

You can start with a new prop. If your numbers are correct your theoretical speed is 48 MPH. With a top end of only 33-34 mph your looking at 31% prop slip. Should be a lot less slip

that must be an empty boat with just an operator in it, i assume. now im curious, i really need to take a gps on mine and see how fast it is. doesnt seem close to 50, but maybe it is. my perception of speed on water might be off a bit...

 

[dingbat]

Re: Any ideas on how to pick up speed?

that must be an empty boat with just an operator in it, i assume. now im curious, i really need to take a gps on mine and see how fast it is. doesnt seem close to 50, but maybe it is. my perception of speed on water might be off a bit...

Theoretical speed has nothing to do with loading. It's simply multiplying the prop?s pitch (travel per rev) times the RPM divided by the gear ratio of the lower unit to determine how fast you would travel if the prop had zero slip. In reality you?ll never get to zero slip do to various reasons, but some props will get you a whole lot closer than others.

http://www.mercuryracing.com/propellers/propslipcalculator.php

 

[5150abf]

Re: Any ideas on how to pick up speed?

There are 2 ways to gain speed on the water, lose weight or increase power, that is really it.

You may tinker or get a new prop but you will probably only see 1-2 mph increase at best.

that is a pretty heavy boat and a small motor so I would say you are doing pretty well now.

 

[solar7647]

Re: Any ideas on how to pick up speed?

You can start with a new prop. If your numbers are correct your theoretical speed is 48 MPH. With a top end of only 33-34 mph your looking at 31% prop slip. Should be a lot less slip

My numbers came up with the same Theoretical speed. I was thinking that i am getting to much slip too. Every thing I read said that 10% lose in slip is good and less then 10% is great. But I have great bite, I will adjust the pitch a little and see what I get. I will try to see how much dead weight i have and can get rid of. Thanks

 

[solar7647]

Re: Any ideas on how to pick up speed?

isnt 33-34 mph about all that engine will do in a 21ft hull? that seems pretty acceptable to me. heck, i dont even know if my 19 with a 305 in it does over 40.

I just helped my brother get the same boat as yours as a project boat, same engine and all. We have almost gotten it to 10% of the Theoretical top speed we are at 16% right now with a old 19 inch pitch prop. we are hitting 47mph according to GPS.

 

[Bob_VT]

Re: Any ideas on how to pick up speed?

I think you have too much pitch. I had a 24' cuddy with a 170/470 and with a 15 x 15 prop I would hit 38 by GPS trimmed out. It was a heavy boat too.

 

[redone4x4]

Re: Any ideas on how to pick up speed?

I just helped my brother get the same boat as yours as a project boat, same engine and all. We have almost gotten it to 10% of the Theoretical top speed we are at 16% right now with a old 19 inch pitch prop. we are hitting 47mph according to GPS.

oh I believe it. Everyone else that rides in my boat says its pretty fast for what it is, it just doesnt seem that way to me. I have a 14 1/4 x19 on it. So I can expect 40+ out of her, that would be great. Hopefully I go out in a few weeks n remember the gps haha!

Thats why I would think that with an even bigger boat like yours, but half the engine, 35 mph would be pretty respectable. I dont have much experience in changing pitches though, so maybe 40+ is reasonable? i dunno. regardless, thanks for the info. lets see pics of your brothers boat lol.

 

[WTR4FUN]

Re: Any ideas on how to pick up speed?

I just helped my brother get the same boat as yours as a project boat, same engine and all. We have almost gotten it to 10% of the Theoretical top speed we are at 16% right now with a old 19 inch pitch prop. we are hitting 47mph according to GPS.

47...that's impressive, what is the hp rating for that motor?

 

[MikDee]

Re: Any ideas on how to pick up speed?

I just helped my brother get the same boat as yours as a project boat, same engine and all. We have almost gotten it to 10% of the Theoretical top speed we are at 16% right now with a old 19 inch pitch prop. we are hitting 47mph according to GPS.

If you look at my signature link, you'll see my old 79'- 20' SeaRay bowrider with a 305/228hp 4bbl (5.0) Mercruiser with thru hull exhaust, & a 14"X19" black max alum prop that would do 48mph, fully trimmed out, (one time I hit 50mph with a brand new version of the same prop)

 

[redone4x4]

Re: Any ideas on how to pick up speed?

47...that's impressive, what is the hp rating for that motor?

my 305 is rated at 230 hp, it sounds like the same one his brother has. now im lookin forward to a 1-man speed run lol

 

[MikDee]

Re: Any ideas on how to pick up speed?

my 305 is rated at 230 hp, it sounds like the same one his brother has. now im lookin forward to a 1-man speed run lol

Can you get 4600rpm? If so you should be there with the 19" prop.

Here's a simple accurate formula I picked up along the way:


RPM, divided by the gear ratio = prop speed, times the prop pitch -1 = speed
4600 divided by 1.5 ratio = 3066 X 18" (19" -1) = 55.18 theoretical speed minus 10% slip = 49.66 mph actual,,, Or, minus 15% slip = 46.90 mph (you hafta estimate what the % slip may be usually between 10-20%)

 

[redone4x4]

Re: Any ideas on how to pick up speed?

i can get over 4000 rpm, but never tried it all out, as far down as it would go with it. If it does over 40, im pretty happy with it anyways. the 19 pitch prop has good speed and pulls skiers and tubes pretty well...i didnt expect that much of a balance out of it. i figured it would be one or the other.

Edit: i thought mine has the 1.68 ratio (still not sure), which would put it well over 40 mph, even at 4000 rpms.

 

[solar7647]

Re: Any ideas on how to pick up speed?

The 5.0 that is on the Capri that I am helping my brother with has a Cobra with a 1.5 ratio on it, I would expect that yours would to. You need to get her out and ramp it up to 4600rpm! They really scream.

 

[TheLucille]

Re: Any ideas on how to pick up speed?

I read several places that if the transducer for the depth find is to close to the drive in will increase cavatation, do you think moving out to the side would help. I am currently topping at 33-34mph according to GPS. Thanks

Lots of good suggestions, just wanted to clarify this part of your question: The prop/hull causes cavitation that causes the transducer to misread, the transducer doesn't usually cause problems for the prop It should not be directly in front of the drive/prop though or it may cause slight issues, nothing that'll pick up 3-4+ mph by fixing.