Regarding how the performance--which is probably most characterized by the amount of time it takes for the hull to transitions to hydroplane mode from displaement mode and the ultimate maximum boat speed possible obtained--varies when the number of persons aboard is increased to eight persons from one person:
For the sake of an example, let us assume that the 190 MONTAUK boat with engine, fuel, required safety gear, and general accessories might weight about 2,795-lbs. This is a reasonable assumption. It is based on the reported weights from a Boston Whaler boat performance test of the 190 MONTAUK with a 115-HP engine.
Let us assume that each person added weights 180-lbs. This is a reasonable assumption for an adult male, and, in fact, might be a bit on the low side of real weight.
With one person added, the boat weight increases to 2,975-lbs. We can refer to this as the baseline boat weight.
Now we can compute the power-to-weight ratio: 115-HP/2975-lbs = 0.038-HP/lbs or about 25.9-lbs-per-HP.
Boston Whaler reports the boat with two people at total weight 3,125-lbs can hit 41-MPH. The power to weight ratio for that is 115/3125=0.0368. From this we can calculate a hull factor constant: c = 214.
With this information we can predict how performance will change as more people are added. Let's go for the worst case, there are eight adults on the boat. We have to add seven more people at 180-lbs each, or 1,260-lbs, and add that to the baseline boat weight, 2,975-bs. The boat with eight people should weigh about 4,235-lbs. Now the HP-to-weight ratio becomes smaller:
115-HP / 4235-lbs = 0.027
Using the speed prediction method as above, the top boat speed should now be:
MPH = 214 × 0.027^0.5
MPH = 35
Now we must assess what a change in top boat speed means when the boat speed decreases to 35-MPH from 41-MPH. Can we describe this 6-MPH decrease in top speed as being characterized as "horrible performance"?
We can look at this another way. What increase in horsepower would be needed to maintain the boat at 41-MPH with the load of eight passengers? This can also be calculated easily. We just need to maintain the same power-to-weight ratio as before. The 41-MPH boat speed with two aboard occurred with the power-to-weight ratio of 0.0368-HP/lbs. To maintain that ratio at 4,235-lbs of boat and passengers, we need 4235 × 0.0368 = 156-HP.
The 190 MONTAUK is only rated for 150-HP maximum, so we cannot compensate for seven additional passengers and achieve the 41-MPH we got with the 115-HP engine and only two people aboard.
None of this is particularly surprising to anyone who has operated a small boat. Because small boats do not weigh very much, the addition of seven adult passengers increase the boat weight by 1,260-lbs
and is certain to have a negative effect on boat performance. To look at the likely boat speed reduction--which has been characterized as being "horrible"--and to find the top speed might reduce by 6-MPH seems quite tolerable as opposed to "horrible."
The notion of having eight people aboard a 19-foot boat should also be considered. If the goal in buying a boat is to be able to operate the boat on a continual basis with eight people aboard, there are probably some other considerations that might need to be taken beside the reduction in top speed by 6-MPH to 35-MPH from 41-MPH with only two aboard.
It is important to note that in the speed predictions above, there is no allowance for a propeller pitch. We just assume that the propeller pitch is suitable for allowing the engine to accelerate to its rated engine speed for maximum horsepower. In real world outcomes, it will be very likely that a propeller that permitted the 115-HP engine to accelerate to 6,000-RPM with one or two people aboard may no longer allow the same engine to accelerate to that same speed when eight people are aboard. A change in the propeller may be necessary to get optimum performance with eight people aboard. If the propeller pitch were not reduced, the performance of the boat, having been tuned to optimum for a much lighter load, would likely be less than optimum.