can any one help me to understand

[mannydvk11]

hello i have a 1996 larson hantom with a 5.7 mercruser I/O with and alpha one on it i have a 15 X 19p aluminum prop 3 blades today it was really nice day,lnice and flat so i tested the speed of the boat @ WOT my gps gave me 38.9 MPH @ 6000 RPM i had a full tank of gas and it was just me and my girl in the boat i was not able to hit more then 38.9 my Q? is now what type and what size prop will be good over all for my boat also i know-this when i am going at slow speed in the inter coastal i travel at 3000 rpm and the boat feels like is dragging

 

[ewenm]

Re: can any one help me to understand

mate from my understanding that 6000 rpm is way to much, i thought 4800 was about it for your motor, and again from my understanding i think you can handle some more pitch, which will make your top speed a little better,and bring the wot rpm down, thats of course depending on if your tacho is correct.you probably want to test that first, pick up another tacho and connect it up and see what it gives you.

at 6000 rpm it must have sounded like it was screaming

 

[tazrig]

Re: can any one help me to understand

Around 4800 - 5000 is about max for a 5.7 For every 1 inch of pitch you add you can expect to loose approximately 150-200 rpm's. (and get more bite out of the water) So if you are turning up 6000 wot (way to high) with a 15x19 you should go to a 15x24 or 15x25 which would drop you down to approx 5000 rpm at WOT. (or slightly lower) This would give you more speed assuming when you hit 6000 rpm now you're not just experiencing cavitation.

 

[bruceb58]

Re: can any one help me to understand

I would double check your tach with a good shop tach before I went out again to verify if you are seeing the correct RPM.

From your other posts you state your boat is 23.5'. The 19 inch pitch prop is probably the correct pitch. I think your tach is giving you a wrong reading.

 

[steelespike]

Re: can any one help me to understand

Seconded, the tach is wrong.
If you were turning 6000 rpm with a 1.47 gear ratio,19" prop assuming 10% slip you would be doing 66 mph.
Tell us more about your boat. Is Hantom a correct spelling?We'll need your gear ratio.Probably stamped on the outdrive.

 

[mannydvk11]

Re: can any one help me to understand

mate from my understanding that 6000 rpm is
way to much, i thought 4800 was about it for your motor, and again from my understanding i think you can handle some more pitch, which will make your top speed a little better,and bring the wot rpm down, thats of course depending on if your tacho is correct.you probably want to test that first, pick up another tacho and connect it up and see what it gives you.

at 6000 rpm it must have sounded like it was screaming

Thanks for u answer ewenm I will double check the tachometer but according to the guy who sold the boat is practical new

 

[mannydvk11]

Re: can any one help me to understand

Around 4800 - 5000 is about max for a 5.7 For every 1 inch of pitch you add you can expect to loose approximately 150-200 rpm's. (and get more bite out of the water) So if you are turning up 6000 wot (way to high) with a 15x19 you should go to a 15x24 or 15x25 which would drop you down to approx 5000 rpm at WOT. (or slightly lower) This would give you more speed assuming when you hit 6000 rpm now you're not just experiencing cavitation.

tazrig the boat feels like is dragging when am in slow speed I feel like I have to push it to 3000 rpms to move thanks for you answer

 

[mannydvk11]

Re: can any one help me to understand

I would double check your tach with a good shop tach before I went out again to verify if you are seeing the correct RPM.

From your other posts you state your boat is 23.5'. The 19 inch pitch prop is probably the correct pitch. I think your tach is giving you a wrong reading.

I will try to buy one ASAP and see if there is any differences and the correct spelling is hamton lol sorry about that.

BTW guys I am thinking on changing the prop any time now what is the differences if I get a 4 instead of 3 I want to be able to move nice and smooth on idle and get good speed

Thanks guys for the replays

 

[bruceb58]

Re: can any one help me to understand

Once you get the tach figured out and to see what RPM you are actually running at, you will know better.

In the short term, look at the back of the tach and make sure the selector on the back of it is pointing at 8 cylinders. If it is not, you may actually be running at too low of an RPM at WOT and will actually need to drop pitch. For example, if it's set to 6 cylinders, your actual RPM may be 4500 which would be about right.

Personally, a 3 blade prop is fine and a 4 blade will actually make you go slower because of the increase in drag.

 

[jestor68]

Re: can any one help me to understand

It's a Larson Hampton. They came in 23 and 24 ft versions. It's dry weight is 4200 lbs. As others have posted; check the tachometer. Your numbers through the Mercury prop selector come up with a 15.5 X 18 pitch, which is basically the same as the prop you have.

 

[Tail_Gunner]

Re: can any one help me to understand

I would double check your tach with a good shop tach before I went out again to verify if you are seeing the correct RPM.

From your other posts you state your boat is 23.5'. The 19 inch pitch prop is probably the correct pitch. I think your tach is giving you a wrong reading.

Get yourself a flashlight ..crawl up under the dash and peek at the tach. There is a switch there some place (back of tach) that has three setting's 4..6..8 make sure it's set to 8...Actually switch back and fourth a few time's just to knock off any corrsion that may have taken place.

 

[mannydvk11]

Re: can any one help me to understand

It's a Larson Hampton. They came in 23 and 24 ft versions. It's dry weight is 4200 lbs.View attachment 179544 As others have posted; check the tachometer. Your numbers through the Mercury prop selector come up with a 15.5 X 18, which is basically the same as the prop you have.

That is the same one I have with out the window in the front

I will check when I get home today

Now lets say the numbers in my prop are close to what am suppost to what will be a normal speed for this boat can I improve on it 38 mph

 

[bruceb58]

Re: can any one help me to understand

Now lets say the numbers in my prop are close to what am suppost to what will be a normal speed for this boat can I improve on it 38 mph

Assuming you can get up to the proper WOT RPM, probably not.

When was the last time you did a complete tune up on the engine...plugs, cap, rotor...

 

[Tail_Gunner]

Re: can any one help me to understand

Well a 1.51 gear ratio with a 19p prop 4800 rpm 12% slip get's 50mph....With a 260 hp 5.7 4000lb boat...Hmmm THis boat is new to you right....If so tuneup and verify the tach..i would think a 17p prop would net 45...Dunno that boat but it seems right.

 

[mannydvk11]

Re: can any one help me to understand

Assuming you can get up to the proper WOT RPM, probably not.

When was the last time you did a complete tune up on the engine...plugs, cap, rotor...

i did the tune up around November i even have a complete ignition cables oil filter water separator in the boat just in case something happens

 

[mannydvk11]

Re: can any one help me to understand

ok i check on my boat and i have a teleflex tachometer when i search online i found this http://www.jamestowndistributors.co...TMTechCat-Instruments(v4_0)Tech Reference.pdf look in page 340 and 341 this is what it say

Tachometers (Inboard & Stern Drive 4-Stroke
Engines – Standard Ignition)

1. Tachometer will not operate on a MerCruiser
with Thunderbolt I (early 1970’s era, but will
operate on subsequent models) or an IndMar LT-1 electronic ignition. Tachometer will operate with
other OEM electronic engine ignition systems.
Teleflex has not tested after market systems and
cannot guarantee their compatibility.

this is my engine

 

[bruceb58]

Re: can any one help me to understand

So did you look at the back of the tach to see if the selector is pointing at the correct number of cylinders?

 

[bruceb58]

Re: can any one help me to understand

So did you look at the back of the tach to see if the selector is pointing at the correct number of cylinders?

The same page 340 shows what it looks like

 

[mannydvk11]

Re: can any one help me to understand

So did you look at the back of the tach to see if the selector is pointing at the correct number of cylinders?

The same page 340 shows what it looks like

yes sorry i was uploading some pics this is what shows

 

[bruceb58]

Re: can any one help me to understand

I would turn that selector back and forth a couple times, like Tailgunner said, just in case the switch is not making a good connection.

Out of curiosity, what RPM does the tach say your engine is idling at?