2 persons vs. 8 persons aboard

peterjaw

peterjaw

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Good day , captains,
My boat is Quicksilver 855 weekend with single 300 HP outboard.

If I have 2 adults aboard, I can reach 31 knots, however, when there are 8 adults aboard, I can only reach 22 knots.

If everything else be equal, is this normal?
 
It’s normal! Weight is everything in a planing boat. But sometimes you can distribute your passengers better, for example if you make them sit up front that can help the boat get on plane and go faster.
Make sure you have enough life jacket for everyone aboard.:encouragement:
 
Bouba said:
It’s normal! Weight is everything in a planing boat. But sometimes you can distribute your passengers better, for example if you make them sit up front that can help the boat get on plane and go faster.
Make sure you have enough life jacket for everyone aboard.:encouragement:
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Thank you very much, Bouba, your explanation makes me feel better.

4 of them sat in aft area, and the other 4 were in the wheelhouse.

As regarding to the life jacket, no worry, I have 10 in my boat.

Thank you very much.
 
Personally would be very very careful indeed about taking that boat very far with 10 people on board....and.....yes have read the sales advertising fluff ?

" Suited for 10 persons you can take it out to the sea,"....Hmm.
 
oldgit said:
Personally would be very very careful indeed about taking that boat very far with 10 people on board....and.....yes have read the sales advertising fluff ?

" Suited for 10 persons you can take it out to the sea,"....Hmm.
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This is my first boat so I am still learing.

In fact, I feel confused about some advertisements of bigger boats (and my boat).
I mean, my boat is smaller but the manufacturer say 10 persons, but why some bigger boats can only
have 6 or 8 persons?

How do they determin the number of person to take?

Despite the safety reason, I think it is more comfortable to have utmost 5 persons on my boat.
 
peterjaw said:
Good day , captains,
My boat is Quicksilver 855 weekend with single 300 HP outboard.

If I have 2 adults aboard, I can reach 31 knots, however, when there are 8 adults aboard, I can only reach 22 knots.

If everything else be equal, is this normal?
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The magnitude added kg,s say 450 for Ave 8 pax , against 150 kg for two pax depends on the % of added weight by pax and the Hp available to shove it with its lift inducing properties.
If it can’t lift with its Hp much , ie just stays sunk or more sunken then the extra drag of its extra wetted area will kill its ultimate planing speed .

So with abundance of power and a more lifting hull form you don’t really notice a great drop off of speed .

That’s an awfully huge drop btw .
So it’s underpowered and the hull form has ran out of lift tricks .

If it got abundant power and a over generous lifting hull form then all that fully loaded does is dull acceleration a bit and yes a very slight almost imperceptible a knot or so of cruising speed for a given rpm .
What it ( extra kg,s ) does do that is noticeable is improve wave crushing and soften the ride more and depending on exactly where the extra weight is change the trim .Trim change is adjusted out or should be with the tabs to get back at optimal AoA for speed / fuel efficiency/ range etc .
This is the experience I have with my boat adding 3 tons +++ to 17 .

Which in % is probably more than your net 300 / 400 kgs ( or what ever ) going from 2 to 8 pax ?

So for me I would be suspicious of loosing so much speed and you are right to question “ is this normal “
 
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Also bear in mind that people moving about the boat whilst underway in a small boat can have a dramatic change in its handling.

I had a cranchi 33 ( so a few metres bigger ) and I had to ask people not to move or to tell me before hand as an abrupt weight shift could cause it to chine ride which was bordering on prettifying. I would hope your hull is better designed but people moving will make a big difference.
 
I have a smallish boat (7.7m long), but when loaded with full fuel (300litres),full water (70litres) and max people (7),its only marginally slower than it is with 1 person, low fuel, no water etc. Maybe its 2-3 knots slower.

Yours soundslike a big loss in speed, it may be normal or it may not, might be worth checking with the manufacturer.
 
julians said:
I have a smallish boat (7.7m long), but when loaded with full fuel (300litres),full water (70litres) and max people (7),its only marginally slower than it is with 1 person, low fuel, no water etc. Maybe its 2-3 knots slower.

Yours soundslike a big loss in speed, it may be normal or it may not, might be worth checking with the manufacturer.
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Trim of load / engine ?
 
I guess Peters issue could be that the load is not well placed or that the drive and/or trim tabs are not at the optimum position, would need some proper experimentation to find out
 
julians said:
I guess Peters issue could be that the load is not well placed or that the drive and/or trim tabs are not at the optimum position, would need some proper experimentation to find out
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My boat has almost all heavy stuff under the aft deck, including fuel/water tank, plus the 300 HP outboarder. Now with 4 persons sit there, my boat's tail really sinks.

I trimmed my engine all the way in but still failed to get the speed.

I understand some lost in speed, but 1/3 lost is out of my imagination.

That is why I ask "is this normal".
 
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In addition to the engine trim, Have you got trim tabs? If the boat rides bow high then Trim tabs will be able to lift the stern more than just the engine trim on its own.

But normally for max speed you want the trim tabs entirely retracted (so no drag from the tabs) and the engine trimmed out/up to the optimum angle. I've never owned a boat where max speed was achieved with the engine trimmed all the way in/down, they always benefitted from a few degrees of drive/engine trim.
 
If your max speed is ahcived with engine trimmed all the way down/in(ie the boat goes slower if you trim out/up) then that may indicate that you have far too much weight at the back, can you rearrange the weight?
 
This may be stating the obvious, if so apologies, but as you accelerate you will probably need to be fully trimmed down/in just to get the boat moving, but as you get on the plane and over the hump, you should trim out/up as speed increases until you get to the optimum trim angle. If you go too far with the trim you will get some odd handling effects like cavitation or porpoising or chine walking etc.
 
julians said:
This may be stating the obvious, if so apologies, but as you accelerate you will probably need to be fully trimmed down/in just to get the boat moving, but as you get on the plane and over the hump, you should trim out/up as speed increases until you get to the optimum trim angle. If you go too far with the trim you will get some odd handling effects like cavitation or porpoising or chine walking etc.
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Thank you very much, Julians.

I do have trim tabs but I did not know they are also for adjusting the angle.

As the engine trim, I have "ActivTrim" from Mercury and it was working when there were 8 persons aboard. I set it to "maximum load". I saw the indicator showed "0 trim", so I thought that means "all the way in".

I think you are right about the weight distribution on my boat. But I don't know what to do, since all the heavy stuffs are at the back, engine, fuel tank, batteries, water tank... etc.

Appreciate for all your information.
 
Try turning active trim off (I presume it can be turned off) and trimming manually, experiment with different position and see what works, same goes for trim tabs, experiment with them and see what happens.
 
Re trim tabs, they can be used for correcting for a left/right weight imbalance or wind direction, but they are also effective for lifting the stern if you put both tabs down an equal amount. Just experiment with them.
 
peterjaw said:
But I don't know what to do, since all the heavy stuffs are at the back,
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Make a picnic on the bow for half your guests. Tell any young couple that they can use your bed. And get the rest to sit next to you at the helm.
The boat might go a lot faster;)
 
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