fuel consumption/revs/speed

[lionelz]

I am trying to just gauge if these figues sound about right or to high etc. I have a rinker 270 which including swim platform is 30ft. It weighs about 3.5 ton plus 2 adults fuel tanks full (375 ltrs) i have a yanamar 4lha stzp 230hp diesel. The out drive is a bravo 2 i have no idea about what prop i have on. I am having to do about 3000 revs to do between 19-21knts i also use a lot of trim taps down which does get on the plane quicker as soon as i bring them up the speed drops is this normal? To counter this i would have to increase revs. Do others use trim taps in this way as well as to counter any list? Does use of taps in this way effect fuel consumption? The manual says burn at 3000 revs should be about 40 ltrs an hour i have yet to check this as i will do on my next fill up.

 

[BruceK]

Are you not trimming out the leg first before using trim tabs?

 

[Momac]

I don't know the boat or the engine - but seems a bit slow.

Try it without trim tabs adjustment - set the tabs pretty level with the hull. Check the tabs are set evenly while in the marina and don't rely on the gauge on the dash.
Tuck the leg in and the boat gets on the plane. when on the plane leave the leg there or possibly trim out a bit if it feels better .

Maybe the props are not the correct pitch.

 

[lionelz]

Are you not trimming out the leg first before using trim tabs?

I have the leg fully trimmed down i do tend to keep it down unless the bow is rising. So should i just use the engine trim and leaving the trim tabs for list correction?

 

[lionelz]

I don't know the boat or the engine - but seems a bit slow.

Try it without trim tabs adjustment - set the tabs pretty level with the hull. Check the tabs are set evenly while in the marina and don't rely on the gauge on the dash.
Tuck the leg in and the boat gets on the plane. when on the plane leave the leg there or possibly trim out a bit if it feels better .

Maybe the props are not the correct pitch.

I checked all trim tabs before setting of on this trip. Both are level with boat when up.

Does use of tabs increase fuel consumption?

 

[JT57]

You should use as little trim tab as possible as they add resistance in the water. The fact your speed drops when you lift them suggests you are planing bow high/stern low, which will also slow you down. Leave the leg fully in/down and try shifting some weight forward. Are your fuel and water tanks towards the stern, and if so, does the boat ride more level when the tanks are not full?
What does your engine rev to and what is the quoted top speed for your boat/engine set up? Are you achieving that top speed?

 

[BruceK]

I have the leg fully trimmed down i do tend to keep it down unless the bow is rising. So should i just use the engine trim and leaving the trim tabs for list correction?

That does not quite make sence. Trim in to get the bow down. Yes, try and trim the bow with the leg before using the trim tabs or you're just creating excessive drag. Also if you are used to smaller boats and especially in smaller boats with a forward helm (1/3 or so back from the bow then getting on a larger boat where the cockpit helm is more like 2/3s back), then the perceived bow lift is deceptive and you may be having a tendancy to over trim. Look at the bow spray as an indicator if you are unsure.

 

[JT57]

I have the leg fully trimmed down i do tend to keep it down unless the bow is rising. So should i just use the engine trim and leaving the trim tabs for list correction?

Trimming the leg out pushes the bow up, which I don't think you want. To trim the boat perfectly requires the use of the trim tabs and the outdrive tilt but it's not easy and by the time you think you've got it right, there will be a wind change or change in sea state and you will find yourself constantly fiddling around with them to save a thimble full of fuel or gain an extra 0.1 knts. I tend to trim the boat as best I can using the tabs only, leaving the outdrive fully in/down. Once I am happy the boat is planing level then I will raise the outdrive leg one increment at a time to see if the speed increases for the given revs. My boat tends to ride best with legs fully down anyway.

 

[lionelz]

That does not quite make sence. Trim in to get the bow down. Yes, try and trim the bow with the leg before using the trim tabs or you're just creating excessive drag. Also if you are used to smaller boats and especially in smaller boats with a forward helm (1/3 or so back from the bow then getting on a larger boat where the cockpit helm is more like 2/3s back), then the perceived bow lift is deceptive and you may be having a tendancy to over trim. Look at the bow spray as an indicator if you are unsure.

Bruce i am talking nonsense and you pf course are correct.

 

[lionelz]

You should use as little trim tab as possible as they add resistance in the water. The fact your speed drops when you lift them suggests you are planing bow high/stern low, which will also slow you down. Leave the leg fully in/down and try shifting some weight forward. Are your fuel and water tanks towards the stern, and if so, does the boat ride more level when the tanks are not full?
What does your engine rev to and what is the quoted top speed for your boat/engine set up? Are you achieving that top speed?

Top speed is about 30knts. I have had 28.3 out of it and it revs at max at about 3700 my fuel is mid ship my water and waste tanks are both sides. Interesting what you say about drag so im confused now (i have no idea about mechanics and engine) if by lowering the trim tabs i go faster ((no change to engine revs) does that increase fuel consumption. Maybe i should just be happy that i am out on the water and can just about afford it. Also maybe i have asked the question to soon as i should wait till i fill it up on my return to gosport

 

[BruceK]

Sorry Lionel. Didn't mean to offend, if it was. I'm in the same boat with Amanzi and initially not being able to see over the bow clearly as I was used to in the cuddy I was over trimming. Thought you might be experiencing the same.

 

[JT57]

Top speed is about 30knts. I have had 28.3 out of it and it revs at max at about 3700 my fuel is mid ship my water and waste tanks are both sides. Interesting what you say about drag so im confused now (i have no idea about mechanics and engine) if by lowering the trim tabs i go faster ((no change to engine revs) does that increase fuel consumption. Maybe i should just be happy that i am out on the water and can just about afford it. Also maybe i have asked the question to soon as i should wait till i fill it up on my return to gosport

No, that won't increase your fuel consumption. You are going faster for the same revs, which is great. You should however get even better performance if you could lower the bow by shifting weight forward rather than using the tabs (drag).

 

[lionelz]

Sorry Lionel. Didn't mean to offend, if it was. I'm in the same boat with Amanzi and initially not being able to see over the bow clearly as I was used to in the cuddy I was over trimming. Thought you might be experiencing the same.

No offence taken it was me being dumb which happens to be most of time when it comes to technical stuff

 

[lionelz]

No, that won't increase your fuel consumption. You are going faster for the same revs, which is great. You should however get even better performance if you could lower the bow by shifting weight forward rather than using the tabs (drag).

Ah got it do you thing the wife would be happy sat on the bow whilst we are going along? ?

 

[JT57]

������

 

[JT57]

Leave me out of that one !!!

 

[Momac]

Top speed is about 30knts. I have had 28.3 out of it and it revs at max at about 3700 t

I think that may be high revs for your engine indicating prop pitch may not be correct (assuming props are not damaged) allowing the engine to rev to high and not reaching the expected 30 knots - but only based on reading on internet so I may be wrong.
.

 

[lionelz]

Its a new prob and new bottom section of out drive however those figures were from before replacement

 

[rhino_mac]

Lionelz, my Monterey was the same(ish) size as your boat and had the same engine. To get on the plane with a full tank and a couple on board I had to initially use a lot of trim tab (pretty much fully down) and then use full throttle to about 20 knots, then ease the trim tabs back. It would plane without trim tabs but it was a real struggle and my boat was very sensitive to fuel load. If it was full, you had to use trim tabs.

A 20 knot cruise on my boat was the most comfortable speed but it would top out at 30 knots exactly. 20 knots was just under 3000rpm from memory but I read quite a few threads on here saying the tacho's linked to these engines were generally over reading by up to 200rpm. 30 knots was pretty much at the redline and fuel consumption really started going through the roof. Good fun though!

To me it sounds like your boat is fine.

 

[lionelz]

Lionelz, my Monterey was the same(ish) size as your boat and had the same engine. To get on the plane with a full tank and a couple on board I had to initially use a lot of trim tab (pretty much fully down) and then use full throttle to about 20 knots, then ease the trim tabs back. It would plane without trim tabs but it was a real struggle and my boat was very sensitive to fuel load. If it was full, you had to use trim tabs.

A 20 knot cruise on my boat was the most comfortable speed but it would top out at 30 knots exactly. 20 knots was just under 3000rpm from memory but I read quite a few threads on here saying the tacho's linked to these engines were generally over reading by up to 200rpm. 30 knots was pretty much at the redline and fuel consumption really started going through the roof. Good fun though!

To me it sounds like your boat is fine.

Thanks for that nearly bought a monterey it sounds like my boat is acting the same as yours was. I think i was getting concerned at the amount of trim i have been using