not speed improvement between 4700 and 5200 rpm

[sandrin olivier]

Hi,

first just to tell you that my boat is a inboard sunray 1989 with 3.7LX (180hp) and alpha one drive with Thru Hub Exhaust & 15 Tooth Spline, 4-? Gearcase.

it's a 18,5 feet with a mini cuddy ( mostly for storage), it's a pretty heavy boat for it size. the actual blade is a 14? inch 19p. ( i should say the actual blade was...)

the boat run cruising speed of 42-44 km/h ( around 28-30 mph i guess) at 3400-3600 rpm, the top speed is 64 km/h ( around 42 mph) at 4700 rpm . also the fuel economy is not the best at cruising, maybe because of the 4bbl. maybe it's just a feeling as i do not investigated this a lot.

after 4700 rpm i notice almost no difference in speed up to 5200 rpm. i take all my speed with the gps.


the boat plane ok but not as fast as i want, but not terrible too. i have to go very progressive on throttle or i feel the boat will almost sink right away if i push the throttle too fast. this happen even with only 2 person.

this week end, i hit a rock in the lake and the prop lost some blade part. now i wonder for replacement should i look for a different prop. the actual is a 3 blade aluminium quicksilver prop.

my goal is to get the best cruising speed performance ( that mean great fuel economy at the fastest cruising speed). there is many brand and model, it's hard to find what fit me best. i also want a model of blade that can take the most of the hit if this happen again, i mean what is the best hub system.

the other thing, the hit... generaly speaking, with this stern (alpha one) what is the most commun thing to be broken or damaged inside? i know i still fonction because i had to come back to the dock with it ( for like 3-4 min ride)


thanks for helping,

sandrin

 

[sho3boater]

Re: not speed improvement between 4700 and 5200 rpm

I'll guess the standard stainless prop would be best. SS prop will be more efficient and more durable, and a performance prop is not going to help you if it is a heavy craft.

The rpm can do that if you are in the max range for the motor. For example if it is rated at max HP at 5,000 rpm and you can run a range of 4600-5400, then it may well go the same speed at 4800 or 5200. For a max of 5000 you would want to run at that or a little over with an empty boat, up to 5400 empty if you carry a large load all the time so you stay in that range loaded. It is better not to run under max range. Some people do if they need a large prop to troll faster and do not run fast very often. It will stress the motor more to run too low of rpm, and at 4600 or less for above motor it would run slower.

Make sure you know the proper rpm range for your motor.

Just a little nick in the prop will affect mpg and speed, the prop needs to be perfect if you want the best speed and fuel use. SS prop is thinner and that is a large part of it being more efficient. They often give you 1" of pitch better performance, you can run 1" more on same boat over an aluminum prop.

 

[hwsiii]

Re: not speed improvement between 4700 and 5200 rpm

Sandrin, I show your motors reommended WOT RPM should be 4,000 to 4,600 RPM and your gear ratio as 1.7:1, please verify this. If you are able to turn 5,200 RPM with the 19" pith aluminum prop you are now using you need to buy a 21" pith Solas Amita 4 blade prop here at Iboats to redue your prop slip, whih is now at 16% and to inrease your ruising speeds. Going to a 21" pith will redue the RPM about 500 RPM and the 4th blade will eat about another 300 RPM, so the results ought to be about 4,400 RPM and this is at the top of the reommended RPM range. The 4th blade will help keep your hole shot about the same as it is now and will inrease your ruising speeds. Here are my numbers and the prop reommendation.

Prop Slip

Prop hange

H

 

[sandrin olivier]

Re: not speed improvement between 4700 and 5200 rpm

hi,

thanks for the answer. for my engine the WOT is 4400-4800. ( 200 rpm in this LX more others 3.7)

now with the proposition of the 21p 4 blade i fear to overprop it, remember that i get these number with 2 people onboard i dont want to overprop it. the 21p 3 blade might fit, a 22p 3 blade would be an option, but there is not much and they are mostly SS.

i dont want SS because i do often beach landing and i fear that if i hit the bottom like this time the damage to the gearcase will be more invasive.

the michigan wheel vortex has a great price too and feature a urethane hub that is supposed to absorb a part of damage if there some hit.

is all prop egal? is all these feature marketing or is there real difference in aluminium prop.

 

[steelespike]

Re: not speed improvement between 4700 and 5200 rpm

hwsiii is usually right on.If you don't want ss check out the Turning point Hustler.Close to ss performance at al prices.I don't think available in a 4 blade but generally provide excellent overall improvement.

 

[hwsiii]

Re: not speed improvement between 4700 and 5200 rpm

Sandrin, please fill this out.

Iboats Boat and Motor Info
1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make. manufacturer and model of motor
7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H