Question re motor trim

[sporty1]

Hi guys
A mate and I have bought almost identical 481 Quintrex Top Enders. Both with 70hp Yamaha outboards.
His uses three trim bars to reach top speed of 31knots / 59.4 Kph at 6300rpm WOT.
Mine use maximum trim bars about 12 bars to reach identical performance.
The down side is if I drop off the plane it cavitates badly and I need to trm down significantly to keep,going at all.
Is the issue motor height? The only difference is he has a stock Yamaha stainless prop to my standard aluminium prop.
Any suggestions?

[Jeremy]

Not sure what the 'issue' is? Did you buy a fishing boat or a performance boat? Forget about 'trim bars' you should go by feel and sound. Stainless prop can make quite a significant difference. Try a Solas prop. You can return it for money back if you are not happy with it.

[lee8sec]

Swap props for a start & see what the difference is to both boats & go from there. Are the motors set at the same height? Leigh

[sporty1]

Good idea to swap props, I don't think they are exactly the same height mine is perhaps an inch of so higher.

[Moonlighter]

Hi Sporty

Read this. It tells you the correct way to determine if your engine is at the right height, and if you have a suitable prop. These guys are the experts, so read and learn!

http://www.veradoclub.com/smf/index.php?topic=137.0

Also, a stainless prop will nearly always "grip" the water better than an alloy one, all other things bening equal. They will hang on better in turns and when trimmed out.

[Noelm]

As well as trying the prop swap, put a cover over the trim gauge, do NOT get all hung up about trim gauges and how one compares to the other.

[tunaticer]

Your difference in trim guages could well be in the guage head itself, they are electronic and require a certain ohm rating for the loom for them to be 100% accurate, so length of loom and connections are another thing that can come into play.
How similarly are the loads distributed in the boats?
Are the drivers of a similar weight?
Was one boat fighting current more than the other?
The variables are almost endless to give differeing readings

I would trim the motors to how best suits your boat's performance and forget comparing indicators.

[sporty1]

THIS APPLIES TO EVERY ENGINE AND BOAT AND IS NOT VERADO SPECIFIC

Before you start messing with props, you must get the engine height right. New props are WORTHLESS if they are not given the opportunity to do what they are designed for. And your engine height is probably not at optimum now. It is due to a little known fact about how mfr's rig: they "bury" them a little. Most mfrs rig the engine lower than optimum so they can keep themselves out of liability due to spinouts or blowouts. And raising to optimum won't cause blowouts. Forget all that stuff about using rulers and how far the hull is above/below the prop/plate. It's easy to tell if your engine height is right: with normal load (50% gas) ,get someone to take it up to cruise, trim it normal, and go back and look at the big anti vent plate above the prop on the lower end. It should be OUT of the water at speed, getting splashed is ok, but out of the flow. If it's totally dry in all seas, you're probably too high. If it's "buried" under the water, you need to raise your engine and then start testing props. A buried cavplate is like dragging a bucket on a rope behind your boat: total drag that hurts performance and puts pressure (pull) on the transom.


An additional advantage in raising is the powerhead 1 -3 inches higher off the water is it reduces the salt/water mist, that gets in the motor and


Now back to propping....

Someone writes:

“I don’t think I really need to test my props by going to WOT, I don’t run fast and just like to cruise”

A common misconception is if you are not a full throttle user (WOT), that the props that came with your boat are the proper ones. Running at full throttle is not something that any of us do often, but just like your car transmission selects the right rpm, a good prop does that too. Too big a prop and you will be “lugging” the engine in cruise. But the testing requires WOT (wide open throttle) to see if you are propped correctly for long engine life.

Here is how to tell if you are propped correctly. This may rattle you to cavitate your engines but it'll be OK....

1. Load the boat down with 50% gas and a light load for fishing cruising, etc
2. Find a very flat piece of water a mile long and bring your trim tabs all the way up.
3. Trim your engines "in" and take it up to full WOT and start trimming up
4. Keep trimming up SLOWLY a bit at a time, letting the boat catch up to the setting...until the engine starts cavitating - losing speed ( you can't hurt it - all new engines have rev limiters)
5. Now drop it back down a bit until it "bites"
6. Now look at your rpm...- it should be in the top half of the mfr's recommended top end rpm. For example, the Verado’s range is 5800-6400 rpm. Therefore I should be at least 6100, 6250 would be ideal. But don't get hung up here, anything over 5900 rpm is ok, you won't be hurting the motor. A few more hundred rpm may get you a bit more speed though. But less than 5800 is going to do long term damage to the motors. It will cause the motor to add gas to the oil, raising the oil level and diluting the protection the oil gives the motor. Eventually the crank will fail and it will not be warranty.
7. Fill out the PDF sheet below so you can compare after you are completed.

compare the results on a prop slip calculator - at WOT and cruise you should be less than 10%---> go here: http://www.rbbi.com/folders/prop/propcalc.htm

It should be noted that prop changing is not a magic cure-all - all you can do is try to get your props up near the upper range of mfr WOT and that's about it. Too many people think changing props is like adding Nitrous or 50 more HP...not so....it's fine tuning in my opin, and for me never been worth more than a few mph or gph when changing a pitch up or down...However, many props are different.... some are smoother than others..others allow you to dock better ....it's not just all about hard statistics....I personally run a prop that gets a bit worse mpg and less top end but it "feels" better than all the others I tried....Don't get hung up on specs searching for the "holy prop grail".

Size depends on what you find above, but three of the best props for < 35 feet are Merc’s Tempest Plus, the Revolution 4 (4 blade) and the new Enertia. Most boats come with a Mirage plus as it is a good all purpose

Thanks moonlighter I found the above information very helpful.
I will look at the plate next time I am out. In relation to trimming the motor till it cavities I can trim it as far at it goes and it will not cavitate at all. I will continue reading and talk with my mechanic to get more information.

[juggernaut]

Not familiar with the Yamaha's as I have an old Johnson where the trim sensor/lever has some facility for adjustment on the outboard so this would also cause different readings at the gauge if they were setup differently. All in all there could be many factors affecting the trim gauge settings as other have mentioned.

I also find the above quote from the Verado forum interesting in that it suggest to get the motor height right before messing with props. I would have thought select your prop and then set your motor height to suit the prop. This is further evidenced by Moonlighters above general statement that a stainless prop will grip better than an ally prop suggesting you could run your motor higher with a stainless prop.

[Moonlighter]

Juggernaut, the guys from the Verado website know what they are talking aboutwhen they say to sort the height first and then play props. If you think i said anything to suggest otherwise, you either misunderstood me, or you misunderstand how drag works when the engine is set too low.

That fellow Ken on the Verado site owns a company called the Propgods in the USA, and is widely respected as one of the most knowledgeable people on props alive today.

Get the height right first, and then a stainless prop will still work better than an alloy one.

It could be that Sportys engine is set at a different height to his mates boat. Also, as Noel says, trim gauges are notoriously variable so comparing one boats trim gauge readings to another is pretty pointless.

Whilst stainless props do have the advantage of better performance over equiv. alloy ones, the alloy props do have one or two advantages in their favor:

1. Cheaper, by a fair way.
1. Will bend easier if they hit something, thus potentially saving gearbox from damage. Rather break a prop than a gearbox!

Alloy props are favored by users who spend their time up rivers and creeks where sandbars, rock bars, logs etc are common obstructions for these reasons.

In open waters, offshore etc stainless would be favored. Grip better in rough conditions such as crossing coastal bars, which can be pretty important.

Horses for courses.

Cheers

ML

[sporty1]

Hi guys thanks for the responses, I don't just rely on gauges. To reach peak performance I have to trim the motor as fake as it can go. The fact I cannot get it to cavitate by giving maximum trim indicates to me perhaps the motor height needs adjustment. My mate only presses his motor up button 2-3 times whereas I press mine between 12-14 times to achieve the same specs.

[juggernaut]

ML you said a stainless prop will generally grip better than ally prop - no misunderstanding there.

Running the motor higher will result in less drag - again no misunderstanding there.

Therefore based on grip is it therefore plausable that you could run a stainless prop higher than an ally prop to reduce drag and not suffer any loss of grip as a result?

In summary if the prop has higher grip then why not run the motor higher to reduce drag if you can be bothered tinkering with heights?

Sure you can run the motor higher then play with props but logically you aren't optimising the prop selected particularly if speed and fuel economy by reduction of drag is of importance (unlikely in a fishing boat scenario).

I'm not dissing the Prop Gods at all. The Prop Gods are giving generic internet advice that the average boater can achieve himself in a relative short time. Fine tuning will be on a case by case basis which is not easily achieved on the net or over the phone. Most people also couldn't be bothered adjusting heights as props are easier to change and the Prop Gods are in the business of selling props.

Can't find the link right now but recall a major outboard manufacture (Johnson user manual i think) stated to select the prop then adjust the height to suit the prop.

[Moonlighter]

Sporty, i think that your plan to check motor height and adjust if necessary is the right one. If you can trim it a long way up without it losing grip with the alloy prop does suggest the engine might be buried too deep.

Do the test as per Verado instructions. A trick is to put a red sticky dot on the trailing edge of the anti-vent plate so its easier to see with water splashing about the place.

Its pretty easy, especially with smaller engines such as a 70 Yammy, to adjust the height. Here's how i did my 70 Yammy 2 stroke on my previous boat:

1. Get 2 helpers! With a bit of muscle and some finesse about them!
2. On a flat piece of ground, chock trailer wheels. Set trailer so its roughly level. Trailer not to be connected to car.
3. Place slab of wood on ground under skeg. Lower engine to vertical position, with skeg close to touching block of wood on ground. Maybe need to put a couple of bricks under wood. Loosen engine bolts but dont take them all the way off yet. Mates hold engine steady.
4. Go to front of trailer and wind jockey wheel up, which lowers the back of the boat, until the engine weight is being solidly taken on skeg thats sitting on wood. Skeg easily strong enough to support engine weight like this.
5. Remove nuts holding engine to transom, and tilt motor backwards carefully just far enough so it slides off the bolts. Mates hold motor steady again! Clean up sika around base of bolts on transom.
6. Go to jockey wheel again, and wind it down (transom is now lower than before) so the transom bolts line up with the set of holes in the engine bracket that you want to use. Bung a good dollop of some sikaflex 291 on each of the bolts as close in to the transom as possible.
7. Tilt motor forward and slide it back onto the bolts. Tighten up. Remove block of wood etc.

Done.

Juggernaut - glad there is no misunderstanding. I also agree that various different profile props, whether stainless or alloy, will create more or less stern lift and will have different grip characteristics and will therefore allow engine heights to be adjusted accordingly.

I dont know why Ken from Propgods would advise people to sort out engine height first before thinking about changing props if their motivation is purely to sell more props. It might even lose them sales!

Need to keep this in perspective. We are talking a 70 Yammy here on a pressed alloy hull, not exactly what you'd call a high performance hull! (No offense Sporty!)

So i doubt if Sporty is likely to be interested in fancy 4 blades or high end Enertia or similar props where it becomes a case of seeking out the final 1 or 2% of performance.

The way I read it, he is just trying to optimise his current situation. And adjusting engine height, as set out above, takes less than an hour on a rainy or windy day, and the cost is limited to a tube of sika! Lot less expensive than even an aftermarket alloy prop!

Cheers

ML

[sporty1]

Thanks ML
As the boat s only 3 months old perhaps the dealer will raise the motor for me as after sales service? Otherwise a couple of heavy helpers with finesse will be the go! Will keep you all informed.

[juggernaut]

ML my comments weren't directed at Sporty's circumstances but at the Verado comment to set motor height before messing with props.

In any case, and as you've pointed out, Sporty probably isn't interested in other props, he has selected his prop and now about to optimise his engine height for that prop.

Isn't this the method I (and outboard manufactures) outline - 1. select prop 2. optimise height for the selected prop?

[Jeremy]

No, because if you change engine height you then need to select the prop to give max rpm at WOT. Prop already on might be OK or might not.

Sporty - your dealership will do this for you at no charge (or should) as part of after sales service on 3 month old boat.