Twin outboard speed trials.

paul1 · 21 October 2007, 20:44

Out in the solent today, my brother and i experimented with the maximum speed reached on each engine running solo . Engines are 40hp Mariners top speed with for each engine was about 25mph, which means they seem to be set up equally, however maximum speed reached with both engines was only another 10mph, obviously acceleration and pulling power is greatly improved, and we like the directionally stability when using both engines, But does the top speed sound about right, would the boat go quicker with smaller props, or larger props ?
also when run together the engines i'm sure rev higher( no rev counter to confirm). Would appreciate any thought's on this.
Regards Paul.

Andy Moore · 21 October 2007, 21:08

Would you go quicker with 1 x 80hp? Half the weight and drag.

Mollers · 21 October 2007, 21:11

Quote:

Originally Posted by paul1

Out in the solent today, my brother and i experimented with the maximum speed reached on each engine running solo . Engines are 40hp Mariners top speed with for each engine was about 25mph, which means they seem to be set up equally, however maximum speed reached with both engines was only another 10mph, obviously acceleration and pulling power is greatly improved, and we like the directionally stability when using both engines, But does the top speed sound about right, would the boat go quicker with smaller props, or larger props ?
also when run together the engines i'm sure rev higher( no rev counter to confirm). Would appreciate any thought's on this.
Regards Paul.

It's all about gearing. Strap two minis together and run 'em down a track, they're not gonna do 150mph. The ONLY advantage of running a pair is safety.

DM · 21 October 2007, 21:16

Quote:

Originally Posted by paul1

top speed with for each engine was about 25mph

That sounds high for one engine on that set up. Did you go from a standing start on one engine or get up to planing speed and then throttle back one engine.

Quote:

Originally Posted by paul1

when run together the engines i'm sure rev higher

They will do. Running on just one engine you're overpropped.

Quote:

Originally Posted by Andy Moore

Would you go quicker with 1 x 80hp?

Yes. About 3/4 knots

paul1 · 21 October 2007, 21:21

Makes sense about the gearing, not in the position to change the engines just yet, so far they have been reliable if not a bit juicy to run, from the safety aspect more often than not, out with young children, so that is good to have, what speed would i expect with one 80 hp as suggested though.
Paul.

DM · 21 October 2007, 21:24

Quote:

Originally Posted by Mollers

The ONLY advantage of running a pair is safety.

Not forgetting better roll stability and increased slow speed manouvrebility

paul1 · 21 October 2007, 21:25

one engine up, and throttled from standstill, with 2 adults and 2 small children.

Andy Moore · 21 October 2007, 21:27

Quote:

Originally Posted by Mollers

The ONLY advantage of running a pair is safety.

Two engines look more sexual.

Mollers · 21 October 2007, 21:30

Quote:

Originally Posted by paul1

Makes sense about the gearing, not in the position to change the engines just yet, so far they have been reliable if not a bit juicy to run, from the safety aspect more often than not, out with young children, so that is good to have, what speed would i expect with one 80 hp as suggested though.
Paul.

If you're gonna go to the bother of re-engining you might aswell go to max design hp. It'll say on the plate what that is. 5.3-5.5m ribs are usually around 90hp.

Downhilldai · 21 October 2007, 21:31

Quote:

Originally Posted by Mollers

The ONLY advantage of running a pair is safety.

Yes, you're quite right there. If one engine breaks down, you'll have an available source of spare parts on the other one

Mollers · 21 October 2007, 21:33

Quote:

Originally Posted by Andy Moore

Two engines look more sexual.

A pair of Mariner 40's?!

Does the thought of Ann Widdecombe get you into a bit of a lather also?

Mollers · 21 October 2007, 21:34

Quote:

Originally Posted by Downhilldai

Yes, you're quite right there. If one engine breaks down, you'll have an available source of spare parts on the other one

Or use it as an anchor!

DM · 21 October 2007, 21:35

Quote:

Originally Posted by paul1

one engine up, and throttled from standstill, with 2 adults and 2 small children.

25mph sounds really high. How were you measuring your speed.

Lots of low power twin set ups would struggle to get on the plane on just one engine.

Mollers · 21 October 2007, 21:38

Quote:

Originally Posted by davidmanning

25mph sounds really high. How were you measuring your speed.

Lots of low power twin set ups would struggle to get on the plane on just one engine.

Are you sure you don't have one 40 and a 4hp?

paul1 · 21 October 2007, 21:39

With a GPS, it was calm though and most of the fuel was proberley gone.

9D280 · 22 October 2007, 10:49

Sounds like you have relatively fine pitched props on there.

One vs two if the revs went up whilst running both, you could probably load the engines a bot more, up your props' pitch & get some extra speed, however you might be gubbed should one break down, as it would be way over propped for one engine alone.

I reckon if you go twin for safety, there's no point in doing so if you can't get on the plane. Like all things, you pays your money, you makes your choice.

Locozodiac · 24 October 2007, 22:21

If you would like to obtain the best acceleration & final speed in your rib, you must perform the WOT (Wide Open Throttle) Test. you must install one tachometer for each engine. There are some induction hour meter/ tachometers available in the market for free battery operated engines ( Rope Start) and electrical tachometers for battery operated engines.

With one person and 2 gallons of gasoline per engine, make a full throttle acceleration run , see how many revs per engine is shown in each tachometer. According to the Mariner Manual your 40 HP long tail assuming they are 2 strokes, must work proper between 5,200 and 5,800 rpm when in full throttle.

The ideal is to have both engine working at 5,800 rpm at Wot. Get same diameter prop delivered with the engine, but with different prop pitch size and perform WOT rpm tests. runs. For any – pitch size there are 200 more revs, for any + pitch size there are 200 less revs. Have not specified if it’s a 2 or 4 stroke engine.

You can ask your engine dealer what pitch alternatives are there to your present diameter 40 HP prop. Try to match the picked prop to match 5,800 rpm or as close to this lecture in full Wot per engine. Never pass the maximum rpm specified or you will be over reving your engine.

Regards,
Locozodiac

iainmartins · 25 October 2007, 00:30

Quote:

Originally Posted by Mollers

Are you sure you don't have one 40 and a 4hp?

My friend has a 5.5 rib with 2 mariner 40's top speed 29 knts and can't get on the plain with one engine.

9D280 · 25 October 2007, 10:27

But I've driven a SR5.4 with 5 bods aboard on a single 30 - got on the plane no problem.... (eventually - but most of the wait was down to loosing the water ballast!).

Like I say, you need to decide why you want twins, and if you want to get back on the plane with one dead or not. Bottom line is that to get home on one, you'll need to drop your prop pitch, killing your top speed....

codprawn · 25 October 2007, 19:05

Quote:

Originally Posted by iainmartins

My friend has a 5.5 rib with 2 mariner 40's top speed 29 knts and can't get on the plain with one engine.

You are supposed to use them in the water that's why!!!